The interpretation timeline
1 Corinthians 9:20 Douay-Rheims

And I became to the Jews, a Jew, that I might gain the Jews:

A.D. 215 – 1871 9 witnesses 10 excerpts

Patristic

before A.D. 750
215
A.D.
c. A.D. 150–215
“"For not only for the Hebrews and those that are under the law "according to the apostle, "is it right to become a Jew, but also a Greek for the sake of the Greeks, that we may gain all.". Also in the Epistle to the Colossians he writes, "Admonishing every man, and teaching every man in all wisdom, that we may present every man perfect in Christ.". Let a man milk the sheep's milk if he need sustenance: let him shear the wool if he need clothing. And in this way let me produce the fruit of the Greek erudition. , that evil has an evil nature, and can never turn out the producer of aught that is good; indicating that philosophy is in a sense a work of Divine Providence.”
Source
220
A.D.
Tertullian Patristic
c. A.D. 150–220
“But, inasmuch as our very perverse cavillers obtrude the rebuke in question for the set purpose of bringing the earlier doctrine into suspicion, I will put in a defence, as it were, for Peter, to the effect that even Paul said that he was "made all things to all men-to the Jews a Jew," to those who were not Jews as one who was not a Jew-"that he might gain all." Therefore it was according to times and persons and causes that they used to censure certain practices, which they would not hesitate themselves to pursue, in like conformity to times and persons and causes.”
Source
“Their truth may be inferred from their agreement with the apostle's own profession, how "to the Jews he became as a Jew, that he might gain the Jews, and to them that were under the law, as under the law,"-and so here with respect to those who come in secretly,-"and lastly, how he became all things to all men, that he might gain all." Now, inasmuch as the circumstances require such an interpretation as this, no one will refuse to admit that Paul preached that God and that Christ whose law he was excluding all the while, however much he allowed it, owing to the times, but which he would have had summarily to abolish if he had published a new god.”
Source
407
A.D.
John Chrysostom Patristic
A.D. 347–407
“"And I became," says he, "to the Jews as a Jew, that I might gain Jews." And how did this take place? When he circumcised that he might abolish circumcision. Wherefore he said not, "a Jew," but, "as a Jew," which was a wise arrangement. What sayest thou? The herald of the world and he who touched the very heavens and shone so bright in grace, doth he all at once descend so low? Yea. For this is to ascend. For you are not to look to the fact only of his descending, but also to his raising up him that was bowed down and bringing him up to himself. "To them that are under the law, as under the law, not being myself under the law, that I might gain them that are under the law." Either it is the explanation of what went before, or he hints at some other thing besides the former: calling those Jews, who were such originally and from the first: but "under the law," the proselytes, or those who became believers and yet adhered to the law. For they were no longer as Jews, yet "under the law." And when was he under the law? When he shaved his head; when he offered sacrifice. Now these things were done, not because his mind changed, (since such conduct would have been wickedness,) but because his love condescended. For that he might bring over to this faith those who were really Jews, he became such himself not really, showing himself such only, but not such in fact nor doing these things from a mind so disposed. Indeed, how could he, zealous as he was to convert others also, and doing these things only in order that he might free others who did them from that degradation?”
Source
430
A.D.
Augustine of Hippo Patristic
A.D. 354–430
“You do not require me to teach you in what sense the apostle says, "To the Jews I became as a Jew, that I might gain the Jews," and other such things in the same passage, which are to be ascribed to the compassion of pitying love, not the artifices of intentional deceit. For he that ministers to the sick becomes as if he were sick himself; not, indeed, falsely pretending to be under the fever, but considering, with the mind of one truly sympathizing, what he would wish done for himself if he were in the sick man's place. Paul was indeed a Jew; and when he had become a Christian, he had not abandoned those Jewish sacraments which that people had received in the right way, and for a certain appointed time. Therefore, even although he was an apostle of Christ, he took part in observing these; but with this view, that he might show that they were in no wise hurtful to those who, even after they had believed in Christ, desired to retain the ceremonies which by the law they had learned from their fathers; provided only that they did not build on these their hope of salvation, since the salvation which was foreshadowed in these has now been brought in by the Lord Jesus. For the same reason, he judged that these ceremonies should by no means be made binding on the Gentile converts, because, by imposing a heavy and superfluous burden, they might turn aside from the faith those who were unaccustomed to them.”
Source

Scholastic

c. 1100 – 1500
1270
A.D.
1225–1276
“Above he showed that he does not wish to accept expenses, and this on account of the intention of a reward; here he shows that he did the same thing on account of love of the Gospel. Where first he says that he made himself the servant of all; second, that he strove to accommodate himself to all, there "and I became to the Jews," etc.; third, he subjoins the reason why he did this, there "but all things I do on account of the Gospel," etc. In the first he implies a threefold commendable condition in servitude: which are generality, liberality, usefulness. He serves therefore freely or liberally, there "when I was free," etc.; generally, there "of all... me," etc.; usefully, there "that I might gain the more," etc. He says therefore "for when I was," etc., as if to say: I will set forth the Gospel without expense, for I have done even more, namely that although "I was free from all," etc., that is, obligated by no one's merits, "I made myself the servant of all," accommodating myself to all through lowliness, and as it were constituting myself a debtor — 2 Cor. 4:5: "but we are your servants through Jesus." And this "that I might gain the more," that is, that I might make a gain of my preaching and servitude. For he sought not things, but souls — 2 Cor. 12:14: "I seek not your things, but you." For if it is commendable to serve in temporal things for temporal gain, how much more in spiritual things for spiritual gain? Then, when he says "and I became to the Jews," etc., here he shows that he strove to accommodate himself to all. And first he says that he accommodated himself to those not yet converted; second, that also to those already converted, there "I became to the weak," etc.; third, that generally to all, there "I became all things to all," etc. In the first he first says that he accommodated himself to the Jews; second, that to the Samaritans, there "and to those who are under the law," etc.; third, that to the Gentiles, there "to those who were without the law," etc. In the first, he first touches the accommodation; second, the reason for the accommodation, there "that the Jews," etc. He says therefore: "and I became to the Jews as a Jew," namely by observing certain legal things, as in the distinction of foods, in the circumcision of Timothy, Acts 16:3, in the legal purification, Acts 21:24. But this can be understood in two ways. In one way, according to Jerome, "I became to the Jews as a Jew" through a dispensation of simulation — for he pretended to observe certain legal things, which nevertheless he did not observe. In another way, according to Augustine, "I became a Jew," etc. — for he truly condescended to them in the observance of certain legal things on account of pious compassion; and he did this that he might gain the Jews for Christ, that is, that he might convert them to the faith of Christ. Then, when he says "and to those who are under the law," etc., here he says that he was accommodating himself to the Samaritans. Where three things are noted about him: first, his sagacity in that he accommodated himself; second, his freedom in that he was not under the law; third, his usefulness in that he was gaining them. He says therefore: "I am also to those who are under the law of Moses," that is, to the Samaritans, who are not Jews but Assyrians, who were brought to inhabit the land of Israel, 4 Kings 17:24 ff. These also were under the law of Moses, because they received only the five books of Moses. To these therefore "I became as if I were under the law," namely by approving the law, and by teaching Christ from it, although "I myself was not under the law," according to literal observance, or servilely, because "the law is not laid down for the just man," 1 Tim. 1:9. And I did this therefore "that I might gain those who were under the law," namely the Samaritans themselves, by converting them to the faith of Christ. Then, when he says "to those who were without the law," etc., here he says that he accommodated himself to the Gentiles. Where first he touches his conformity; second, the truth of his faith, there "although without the law," etc.; third, the rectitude of his intention, there "that I might gain," etc. And this is: "and I became to those who were without the law," that is, to the Gentiles — Rom. 2:14: "for the Gentiles, who do not have the law" — "as if I were without law," that is, by assenting to their reasonings, and to the good positions of the philosophers, as is clear in Acts 17; "although I was not without law" — Rom. 7:25: "with the mind I serve the law of God" — "but I was in the law," not the Jewish law, but of Christ, who is God, and not Moses. Concerning which, Gal. 6:2: "bear one another's burdens, and thus you will fulfill," etc. And I did this therefore "that I might gain those who were without the law," by converting the Gentiles to the faith. O happy zealot! Gregory: "there is no sacrifice like the zeal of souls." Then, when he says "I became to the weak," etc., here he shows that he accommodated himself to those already converted. Where first he sets forth the good manner, second the due end, there "that the weak." He says therefore: "I became also to the weak," in faith, "weak," by abstaining from lawful things — 2 Cor. 11:29: "who is weak, and I am not weak?" So does the good physician, who eats the food of the sick man, that he may provoke him to eat, and so heal him. And this I did "that I might gain the weak," strengthening them in the faith. And briefly, "I became all things to all," as if I were of every sect. Therefore it is said below, 10:33: "as I also please all in all things." And this "that I might make all safe." In the ankle-length vestment which Aaron had, the whole world was depicted, Wisdom 18:24. And, 4 Kings 4:34, Elisha contracted himself to the manner of the boy, and thus revived him. And because, according to Boethius, every otherness is discordant, but likeness truly is to be sought, therefore spiritual men, saving the observance of their life and religion, ought to conform themselves to all. Then, when he says "all things I do on account of the Gospel," above he showed that he strove to accommodate himself to all; here he subjoins the reason why he does this, and this is twofold: one on the part of the Gospel, namely that it may have a free course; the other on his own part, namely that he may obtain the promised reward, there "that a partaker," etc. And this is what he says: "but all things I do on account of the Gospel," to be preached without impediment, "that I may become a partaker of it," that is, of the promises which are contained in it — Matt. 5:19: "whoever shall do and teach, he shall be called great in the kingdom of heaven."”

Post-Reformation

c. 1650 – 1900
1771
A.D.
John Gill Reformed
1697–1771
“To them that are without law, as without law,.... Meaning the Gentiles, who, though they were not without the law of nature, nor without many good civil laws, by which the more cultivated and civilized nations among them were governed, yet they were without the written law of Moses; a description of the Gentiles, usual with the Jews; see Rom 2:12. And to these the apostle accommodated himself, as if he was without the law; by conversing with them without any difference; by eating any sort of food with them; by not circumcising Titus, when the Jews would have had it done; and by resisting Peter, when he, by his example, would have influenced the Gentiles to have lived as did the Jews: being not without law to God; or "the law of God", the moral law; for though he was delivered from the curse and condemnation of it, and as a covenant of works, and the ministry of it by Moses, yet not from the matter of it, and obedience to it, as a rule of walk and conversation: and therefore his compliance with the Gentiles was not in anything that was contrary to the moral law; nor did he act as one that had nothing to do with that law, "but" as one that was under the law to Christ, or "under the law of Christ"; that is, the law of love, which obliged him to take every lawful and proper method for the good of such souls: that I might gain them that were without law; to believe in Christ for life and salvation, and to serve the law of God as in the hands of Christ, the only Lawgiver and King in his church; in testimony of their gratitude to him, for the blessings of grace received by him, without having any dependence on their obedience to it, for acceptance with God.”
Source
1832
A.D.
Adam Clarke Methodist
1762–1832
“Unto the Jews I became as a Jew - In Act 16:3, we find that for the sake of the unconverted Jews he circumcised Timothy. See the note on Act 16:3. To them that are under the law - To those who considered themselves still under obligation to observe its rites and ceremonies, though they had in the main embraced the Gospel, he became as if under the same obligations; and therefore purified himself in the temple, as we find related, Act 21:26 (note). After the first clause, to them that are under the law as under the law, the following words, μη ων αυτος ὑπο νομον, not being myself under the law, are added by ABCDEFG, several others; the later Syriac, Sahidic, Armenian, Vulgate, and all the Itala; Cyril, Chrysostom, Damascenus, and others; and on this evidence Griesbach has received them into the text.”
Source
1849
A.D.
1774–1849
“I became to the Jews as a Jew. That is, upon occasions, not to hinder their conversion, I practised the ceremonies of their law; though I am not under their law, which is no longer obligatory, but only under the new law of Christ. (Witham)”
1871
A.D.
1871
“I became as a Jew--in things not defined by the law, but by Jewish usage. Not Judaizing in essentials, but in matters where there was no compromise of principle (compare Act 16:3; Act 21:20-26); an undesigned coincidence between the history and the Epistle, and so a sure proof of genuineness. to them that are under the law, as under the law--in things defined by the law; such as ceremonies not then repugnant to Christianity. Perhaps the reason for distinguishing this class from the former is that Paul himself belonged nationally to "the Jews," but did not in creed belong to the class of "them that are under the law." This view is confirmed by the reading inserted here by the oldest manuscripts, versions, and Fathers, "not being (that is, parenthetically, 'not that I am') myself under the law."”
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.