The interpretation timeline
Galatians 2:4 Douay-Rheims

But because of false brethren unawares brought in, who came in privately to spy our liberty, which we have in Christ Jesus, that they might bring us into servitude.

A.D. 220 – 1871 7 witnesses 9 excerpts

Patristic

before A.D. 750
220
A.D.
Tertullian Patristic
c. A.D. 150–220
“So great had been his desire to be approved and supported by those whom you wish on all occasions to be understood as in alliance with Judaism! When indeed he says, that "neither was Titus circumcised," he for the first time shows us that circumcision was the only question connected with the maintenance of the law, which had been as yet agitated by those whom he therefore calls "false brethren unawares brought in." These persons went no further than to insist on a continuance of the law, retaining unquestionably a sincere belief in the Creator. Therefore he says: "Because of false brethren unawares brought in, who came in privily to spy out our liberty which we have in Christ, that they might bring us into bondage, to whom we gave place by subjection not even for an hour." Let us only attend to the clear sense and to the reason of the thing, and the perversion of the Scripture will be apparent.”
Source
“"Liberty in Christ" has done no injury to innocence.”
235
A.D.
Hippolytus of Rome Patristic
c. A.D. 170–235
“"And they watched diligently." And this, too, is to be noted. For up to the present time both the Gentiles and the Jews of the circumcision watch and busy themselves with the dealings of the Church, desiring to suborn false witnesses against us, as the apostle says: "And that because of false brethren unawares brought in, who came in privily to spy out our liberty which we have in Christ Jesus."”
Source
407
A.D.
John Chrysostom Patristic
A.D. 347–407
“"And that because of the false brethren, privily brought in." Here arises a very important question, Who were these false brethren? If the Apostles permitted circumcision at Jerusalem, why are those who enjoined it, in accordance with the Apostolic sentence, to be called false brethren? First; because there is a difference between commanding an act to be done, and allowing it after it is done. He who enjoins an act, does it with zeal as necessary, and of primary importance; but he who, without himself commanding it, alloweth another to do it who wishes yields not from a sense of its being necessary but in order to subserve some purpose. We have a similar instance, in Paul's Epistle to the Corinthians, in his command to husbands and wives to come together again. To which, that he might not be thought to be legislating for them, he subjoins, "But this I say by way of permission, not of commandment." (1 Cor. vii: 5.) For this was not a judgment authoritatively given but an indulgence to their incontinence; as he says, "for your incontinency." Would you know Paul's sentence in this matter? hear his words, "I would that all men were even as I myself," (1 Cor. vii: 7.) in continence. And so here, the Apostles made this concession, not as vindicating the law, but as condescending to the infirmities of Judaism. Had they been vindicating the law, they would not have preached to the Jews in one way, and to the Gentiles in another. Had the observance been necessary for unbelievers, then indeed it would plainly have likewise been necessary for all the faithful. But by their decision not to harass the Gentiles on this point, they showed that they permitted it by way of condescension to the Jews. Whereas the purpose of the false brethren was to cast them out of grace, and reduce them under the yoke of slavery again. This is the first difference, and a very wide one. The second is, that the Apostles so acted in Judaea, where the Law was in force, but the false brethren, every where, for all the Galatians were influenced by them. Whence it appears that their intention was, not to build up, but entirely to pull down the Gospel, and that the thing was permitted by the Apostles on one ground and zealously practiced by the false brethren on another. "Who came in privily to spy out our liberty, which we have in Christ Jesus, that they might bring us into bondage." He points out their hostility by calling them spies; for the sole object of a spy is to obtain for himself facilities of devastation and destruction, by becoming acquainted with his adversary's position. And this is what those did, who wished to bring the disciples back to their old servitude. Hence too appears how very contrary their purpose was to that of the Apostles; the latter made concessions that they might gradually extricate them from their servitude, but the former plotted to subject them to one more severe. Therefore they looked round and observed accurately and made themselves busybodies to find out who were uncircumcised; as Paul says, "they came in privily to spy out our liberty," thus pointing out their machinations not only by the term "spies," but by this expression of a furtive entrance and creeping in.”
Source
749
A.D.
John of Damascus Patristic
A.D. 676–749
“The preposition 'because of' (dia) was put here instead of 'according to' (kata). But the sense is this. The Apostles, he says, did not force Titus, who was uncircumcised, to be circumcised, although this was pointed out by the brethren who were brought in secretly and pressed for circumcision. Indeed he put them in the place of spies because of what is foreign to the truth.”
Source
“That they might introduce us again, he says, into the slavery of the law. This is why elsewhere he says, Christ purchased us from the curse of the law (Gal. 3:13).”

Scholastic

c. 1100 – 1500
1273
A.D.
Thomas Aquinas Catholic
1225–1274
“After the Apostle, in the preceding chapter, commended the authority of the evangelical teaching in itself, now in this chapter he commends it on the part of the other apostles and of himself at the same time. And concerning this he does two things. First he commends the authority of his teaching from the approval of the other apostles; second, from the example of himself and the other apostles, at the place "We by nature Jews, not of the Gentiles," etc. Concerning the first he does two things. First he shows that the other apostles approved his teaching; second he shows that he freely reprehended the other apostles in those things which they said contrary to his teaching, at the place "When Peter had come," etc. Concerning the first he does two things. First he treats of the conference which he had with the apostles; second he intimates what followed from it, at the place "but neither was Titus," etc. Concerning the first he does two things. First he sets down the circumstances of the conference itself; second he sets down the conference itself, at the place "and I conferred with them," etc. As to the first he touches on four circumstances, namely the time, the place, the witnesses, and its motive. Now he describes the time when he says, "then after fourteen years." But against this: the Apostle was converted in the first year after the Passion of Christ, and after three years he went to Jerusalem, and thus there are four; and here he says "after fourteen years" he went again to Jerusalem, and thus there come to be eighteen; and then he found Peter in Jerusalem. And this cannot be, because Peter sat at Antioch for seven years, but at Rome for twenty-five years. And so there would be eighteen (two short of twenty) and seven — which are twenty-five years — before he went to Rome, and at Rome he stayed twenty-five years; therefore Peter would have lived after the Passion of Christ fifty years, which is false: because in the fortieth year from the Passion of Christ, Peter suffered at Rome, as is held in history, which was in the time of Nero. I respond. It must be said that when it is said "then," etc., it is not to be understood that after the three years fourteen more years elapsed before he went to Jerusalem, but that in the fourteenth year of his conversion he went up again. Nor are the seven years, during which Peter ruled the Antiochene Church, to be added on top of these fourteen, because he began to rule before these years. And since Antioch is near Jerusalem, it could be that at some point Peter had gone to Jerusalem, and then Paul found him there. And thus it is gathered from history that after fourteen years Peter came to Rome in the time of the emperor Claudius, and being there for twenty-five years, he completed the number of thirty-nine years, and died in the fortieth year after the Passion of the Lord. Now he says "fourteen" pointedly, to show that he had no need of the instruction of the apostles, if he was without them for fourteen years. But he describes the place when he says "Jerusalem." And he says "I went up," because it is situated on high. Now he went up to Jerusalem, in order to show himself in agreement with the prophecy which says, Isaiah 2:3: "Out of Sion shall go forth the law," etc. He describes the witnesses when he says, "with Barnabas, taking also Titus." Barnabas was a Jew, but Titus a Gentile. With them, therefore, he went up, so that he might have witnesses to his teaching, and so that he might show himself inclining to neither side, neither of the Jews nor of the Gentiles. Deuteronomy 17: "In the mouth of two or three every word stands." But he describes the motive when he says "according to a revelation of God," that is, God revealing it and commanding him that he should go up to Jerusalem. From this it can be gathered that all the acts and movements of the apostles were according to the instinct of the Holy Spirit. Job 37:11: "The clouds scatter their light," etc. Consequently, when he says "and I conferred," etc., he treats of the conference itself, where he does three things. First he sets forth the matter concerning which he conferred; second, the persons with whom he conferred; and third, the cause for which he conferred. The matter concerning which he conferred was the Gospel. And therefore he says, "I conferred with them the Gospel of God," etc. The persons with whom he conferred are the greater and more excellent among the apostles: "but apart with those," etc. But the useful and necessary cause: namely "lest perhaps in vain," etc. As to the first he says, "I went up to Jerusalem," where "I conferred with them," as with friends and equals, the Gospel which I preached among the Gentiles — not that I might learn it, since I was already taught by Christ; not that I might be certified of it, since I am so certain that even if an angel should say the contrary, I would not believe it, as is clear above in chapter 1. But I conferred for two reasons: namely, to intimate the unity of my teaching with the teaching of the other apostles. 1 Corinthians 1:10: "That you all say the same thing," etc. He therefore conferred with them, having as it were the same word as they, yet not holding them as equals. Likewise, to avoid the calumny of others. For the Apostle, because he had not lived with Christ, nor been taught by the apostles, but immediately after his conversion began to preach things that were hateful to the Jews — and especially concerning the calling of the Gentiles, and that the legal observances ought not to be kept — thus, then, he conferred the Gospel. But with whom he did this, he shows by adding, "but apart with those," etc., as if to say: not with all, but with those who were among the others of some authority and weight, namely with Peter, James and John, and the other great ones. Ecclesiasticus 9:21: "Deal with the wise and the prudent," etc. But "apart," etc. — not that he dealt or conferred with them concerning shameful or false things, as heretics do, but because he knew that there were Jews there who calumniated, on account of the fact that he had taught concerning the legal observances. And therefore, lest the truth lie open to calumny, he conferred apart with those who would not calumniate. Proverbs 25:9: "Treat thy cause with thy friend, and discover not the secret to a stranger," etc. Ecclesiasticus 8:21: "Before a stranger do not take counsel," etc. Thus, then, both the matter of the conference and the persons are clear. There follows the cause, which was, namely, "lest I should run, or had run, in vain" — that is, lest I be reckoned to have preached uselessly. Now he calls his preaching a "running," on account of the swiftness of his teaching, because in a short time he preached the Gospel from Jerusalem all the way to Illyricum, and even to Spain. Whence that saying could be said of him, Psalm 147:15: "His word runneth swiftly," etc.; 2 Thessalonians 3:1: "Brethren, pray for us, that the word of the Lord may run," etc. But did he doubt that he was running in vain? It must be said that he did not doubt for himself, but concerning those to whom he had preached — because unless his teaching were firmly held by them, as far as they were concerned he would have run in vain; and therefore he wished to confer with them, so that, when the hearers knew that his teaching agreed with the teaching of the other apostles, and was approved by them, they would hold his teaching more firmly, and thus, as far as they were concerned, he would not run in vain. 1 Corinthians 9:26: "I so run, not as at an uncertain thing." Consequently, when he says "but neither was Titus," etc., he shows what followed from the conference held with the apostles. And he sets down three things that followed from it: namely, that he did not depart from his own position; and that nothing was added over and above to his teaching, at the place "but from those who seemed," etc. Third, that his teaching was approved, at the place "but on the contrary, when they had seen," etc. Concerning the first he does two things. First he shows that he did not depart from his position in a certain particular matter; second he shows that he did not depart from it in any other matter either, at the place "but because of false brethren brought in unawares," etc. He says, then: I say that I conferred with them concerning the teaching of the Gospel in such a way that from this it followed that my teaching and position remained firm — namely, that the legal observances were not to be kept, so that the Gentiles were not compelled to keep the legal observances — to such an extent that not even Titus, who was with me, although he was also a Gentile, was compelled, by their arguments, to be circumcised, but was received by the apostles into fellowship uncircumcised. Whence then a decision was given by the apostles concerning the legal observances not being kept, as is found in Acts 15:28. Now the reason why, after the Passion of Christ, the legal observances ought not to be kept is assigned by Chrysostom as follows: for it is manifest that an instrument which is made concerning some promise or covenant holds only until the covenant and promise be fulfilled; once these are fulfilled, the aforesaid instrument no longer holds in this regard. Now circumcision is a certain instrument of the promise and covenant between God and faithful men; whence also Abraham received circumcision as a sign of the promise, as is said in Genesis 17. And because, the Passion of Christ having been accomplished, the promise was discharged and the covenant fulfilled, therefore after the Passion circumcision no longer holds, nor is it of any effect. Thus, then, it is clear that he did not depart from his position in that he did not permit Titus to be circumcised. Consequently he shows that he did not depart from it in any other matter either, when he says "but because of false brethren brought in unawares," etc. But this text is different in different copies and obscure, and is read thus: you say that you did not permit Titus to be circumcised, but why did you not permit it? Did you not elsewhere permit Timothy to be circumcised, as is read in Acts 16:3? To this the Apostle can respond thus: because at that time, when Timothy was circumcised, circumcision was a matter of indifference, namely whether it was kept or not; but now, when the matter concerned Titus, there was a special question about circumcision, which I was saying ought not to be kept. Whence if I had permitted him to be circumcised, since I myself had settled the question, it would have been done contrary to my own decision, nor was it lawful to raise the question further about this, or to make a difficulty of it, as being already determined. And therefore he says: I say that I not only did not permit him to be circumcised by them, to whom we yielded not even for an hour by way of subjection — namely, that the Gentiles should be made subject to the law. And this on account of those brought in unawares — by the Devil or by the Pharisees — false brethren, who feign themselves to be friends. 2 Corinthians 12: "peril from false brethren." Who — namely, the false brethren — came in unawares into the place where the apostles were, "to spy," that is, to spy out our liberty from sin and the law. 2 Corinthians 3:17: "Where the spirit of the Lord is, there is liberty." Romans 8:15: "For you have not received the spirit of bondage," etc. Below, chapter 4:5: "that he might redeem them who were under the law." Which — namely, liberty — we have in Christ Jesus, that is, through faith in Christ. Below, chapter 4, verse 31: "You are not the children of the bondwoman, but of the free." And for this they came in unawares: that they might bring us back into servitude — of the law and of carnal observances — as before the Passion of Christ, which is not to be done, because "no man can lay another foundation," etc., 1 Corinthians 3:11. And this "that the truth of the Gospel might remain with you" — as if to say: in nothing did we yield to them for this reason, namely lest we give occasion to those who said that without circumcision you could not be saved, which is contrary to the truth of the Gospel that I preached to you. But Ambrose reads it otherwise. For according to what has been said above, it is held that he did not yield for an hour on account of those brought in unawares. From which it follows that, had the false brethren not been brought in unawares, he would have yielded to them concerning the observance of the legal precepts. And therefore it was not on account of this, because he would not have yielded to them on account of this, but on account of the truth itself. Therefore Ambrose says that the text is corrupt, and that the word "neither" there is superfluous. Whence he wishes that "neither" not be there. And then the sense is: I did not permit Titus to be circumcised, but I permitted Timothy to be circumcised on account of the false brethren brought in unawares, in the place where I was with Timothy and the others, who came in unawares, etc. And since they had been unable to do this, they were striving to incite the people to sedition against us. To whom — namely, to the false brethren — for this reason we yielded for an hour to subjection, in the act of circumcision, by circumcising Timothy, there where it is said "that the truth of the Gospel might remain," etc., which holds that neither circumcision avails anything, nor uncircumcision, but faith. Now there was a special reason why Timothy was circumcised and not Titus: because Timothy was of a Gentile father and a Jewish mother, whereas Titus was of Gentile parents on both sides. And it was the Apostle's position that those who had been born of some Jewish parent should be circumcised, but those who had been born entirely of Gentile parents should by no means be circumcised.”

Post-Reformation

c. 1650 – 1900
1771
A.D.
John Gill Reformed
1697–1771
“And that because of false brethren,.... This is the reason why the elders did not insist upon the circumcision of Titus, why he did not submit to it, and why the apostle would not admit of it: had it been left as a thing indifferent, or had it been moved for in order to satisfy some weak minds, it might have been complied with, as in the case of Timothy; but these men insisted upon it as necessary to salvation; they were sly, artful, designing men; could they have gained their point in such an instance; could they have got such a precedent at such a time, when this matter was canvassing, they would have made great use of it in the Gentile churches, for which reason it was by no means judged proper and expedient. These men are described as "false brethren": they had the name, but not the grace, which entitles to the character of "brethren"; they called themselves Christians, but were in reality Jews: at the head of these, Cerinthus, that arch-heretic, is said (b) to be. They are further described as such, who were unawares brought in, who came in privily; into the churches, and into the ministry, into private houses, where the apostles were; or rather into the public synod, where they were convened together about this article of the necessity of circumcision to salvation. Their views, aims, and ends were, to spy out our liberty which we have in Christ Jesus; by which is meant, not a liberty to sin, which is no Christian liberty, is contrary to Christ, to the Spirit of Christ, to the principle of grace in believers, and to the doctrines of the Gospel; but a liberty from sin; not the being of it, but the dominion and damning power of it: that branch of Christian liberty the apostle here chiefly designs is a freedom from the law, both the moral law, as in the hands of Moses, and as a covenant of works, though not from obedience to it as in the hands of Christ, and as a rule of walk and conversation; but from obeying it, in order to obtain life, righteousness, and salvation by it, and from the curse and condemnation of it; and chiefly the ceremonial law, circumcision, and all the other rituals of it, and the free use of all things indifferent, provided the glory of God, and the peace of weak believers, are secured. This liberty is said to be had "in Christ", because Christ is the author of it; it is that with which Christ makes his people free; and such as are made free by him, are free indeed; and is what they come to enjoy by being in him; for by having union to him, they come to partake of all the blessings of grace which come by him, and this among the rest. Now the design of these false teachers getting in privily among the apostles, elders, and brethren, was to make their remarks upon this liberty, to object to it, and, if possible, to break in upon it, and destroy it, and so gain another point, which follows: that they might bring us into bondage; to the moral law, by directing souls to seek for justification and salvation by the works of it, which necessarily induces a spirit of bondage, genders to a state of bondage and involves in it; and to the ceremonial law, by engaging to an observance of circumcision, that yoke of bondage, and of day, months, times, and years, and other beggarly elements, which naturally lead on to such a state. (b) Epiphan. contr. Haeres. l. 1. Tom. 2. Haeres. 28.”
Source
1871
A.D.
1871
“And that--that is, What I did concerning Titus (namely, by not permitting him to be circumcised) was not from contempt of circumcision, but "on account of the false brethren" (Act 15:1, Act 15:24) who, had I yielded to the demand for his being circumcised, would have perverted the case into a proof that I deemed circumcision necessary. unawares--"in an underhand manner brought in." privily--stealthily. to spy out--as foes in the guise of friends, wishing to destroy and rob us of our liberty--from the yoke of the ceremonial law. If they had found that we circumcised Titus through fear of the apostles, they would have made that a ground for insisting on imposing the legal yoke on the Gentiles. bring us into bondage--The Greek future implies the certainty and continuance of the bondage as the result.”
Source
Modern · 1953 →

The in-app commentary runs from the Fathers to the early-modern record, then stops — that's where the public-domain sources end, not where the reading does. For the modern reading, follow the sources directly.