The interpretation timeline
Matthew 18:12 Douay-Rheims

What think you? If a man have an hundred sheep, and one of them should go astray: doth he not leave the ninety-nine in the mountains, and go to seek that which is gone astray?

A.D. 367 – 1871 10 witnesses

Patristic

before A.D. 750
367
A.D.
Hilary of Poitiers Patristic
c. A.D. 310–367
“But by the one sheep is to be understood one man, and under this one man is comprehended the whole human race. He that seeks man is Christ, and the ninety and nine are the host of the heavenly glory which He left.”
407
A.D.
John Chrysostom Patristic
A.D. 347–407
“And to this reasoning He adds a parable, in which He sets forth the Father as seeking the salvation of men, and saying, What think you, If a man have a hundred sheep.”
420
A.D.
Jerome Patristic
c. A.D. 347–420
“Others think that by the ninety and nine sheep are understood the number of the righteous, and by the one sheep the sinners, according to that said in another place, I am not come to call the righteous but sinners to repentance. (Matt. 9:13.)”
604
A.D.
Gregory the Great Patristic
c. A.D. 540–604
“(Hom. in Ev. xxxiv. 3.) This refers to the Creator of man Himself; for a hundred is a perfect number, and He had a hundred sheep when He created the substance of Angels and men.”
735
A.D.
Bede the Venerable Patristic
c. A.D. 672–735
“(ap. Anselm.) The Lord found the sheep when He restored man, and over that sheep that is found there is more joy in heaven than over the ninety and nine, because there is a greater matter for thanksgiving to God in the restoration of man than in the creation of the Angels. Wonderfully are the Angels made, but more wonderfully man restored.”
Source

Medieval

c. 750 – 1100
856
A.D.
Rabanus Maurus Medieval
c. A.D. 780–856
“Note, that nine wants only one to make it ten, and ninety and nine the same to be a hundred. Thus members which want one only to be perfect, may be larger or smaller, but yet the unit remaining invariable, when it is added makes the rest perfect. And that the number of sheep might be made up perfect in heaven, lost man was sought on earth.”
Source

Scholastic

c. 1100 – 1500
1256
A.D.
Thomas Aquinas Catholic
1225–1274
“"What do you think?" Here a comparison is set forth. And first the diligent search is set forth; second, the joy over the sheep found. Hence he says, "What do you think?" Thus it has been said that the Son of Man came to save what had perished, because the shepherd seeks the lost sheep. "If a man has a hundred sheep." By the number one hundred the whole universe of rational creatures is signified: ninety-nine is the same number as nine, but only multiplied, because nine multiplied by ten makes ninety; which number, namely nine, falls short of ten by a unit; hence by these sheep he signifies all rational creatures; John 10:27: "My sheep hear my voice"; Psalm 94:7: "but we are his people and the sheep of his pasture." By the sheep that strayed, the human race is signified. And why did he signify it by the sheep that strayed? Because through one man all strayed; 1 Peter 2:25: "you were like sheep going astray." "Does he not leave the ninety-nine in the mountains?" The text does not read "in the desert," but "in the mountains," as is found in the Greek. This is explained in three ways. First, because these ninety-nine signify the Angels who were left in the mountains, that is, in the heavenly places; Ezekiel 34:13: "I will feed them on the mountains of Israel." Or by the ninety-nine the just are signified, by the lost sheep sinners; and thus he left them in the mountains, that is, in the height of justice; Psalm 35:7: "your righteousness is like the mountains of God." Or by the ninety-nine the proud, by the sheep the humble: hence, "does he not leave the ninety-nine in the mountains," that is, in their pride, and go to seek the one that strayed? Psalm 118:176: "I have strayed like a sheep that is lost; seek your servant, O Lord." Next he deals with the joy: "and if it happens that he finds it," etc. Here too a threefold reason can be assigned. That the Lord rejoices over the good is found in Zephaniah 3:17: "your God will rejoice over you with gladness." If by the ninety-nine the Angels are signified, by the sheep man, the reason is plain, because man was worthy of being restored; Hebrews 2:16: "nowhere did he take hold of the Angels, but he took hold of the seed of Abraham." If by the ninety-nine we understand the just, likewise the reason is plain, because a commander loves more the soldier who falls in battle and afterward always fights manfully, than the one who never fell but always fights half-heartedly. Thus, when someone has sinned, and afterward rises up strongly and always conducts himself manfully, he loves him more; 2 Corinthians 7:9: "I rejoice, because you were made sorrowful unto repentance"; therefore the Lord rejoices more over him, etc., since he is of greater zeal. Yet this is not to be extended to all, because a just man can have so much zeal that he pleases God more than does the penitent. According to the third exposition also the reason is clear, because he rejoices more over the one who acknowledges his sin, as is clear in the case of the publican and the Pharisee. He concludes therefore thus: "It is not the will before your Father who is in heaven that one of these little ones should perish." He says less and signifies more, because his will is that they be saved; 1 Timothy 2:4: "who wills all men to be saved." For if he did not will it, he would not send the Angels. Ezekiel 18:23: "Is it my will that the wicked man should die? says the Lord." "If then your brother sins against you," etc. Here he deals with the forgiving of an offense. And first the order is set forth; second the number, at "then Peter, coming to him," etc. Concerning the first, three things. First he sets forth the secret admonition; second the testimony, at "but if he does not hear you," etc.; third the announcement, at "but if he does not hear them, tell it to the Church." Concerning the first, first he gives his teaching; second he assigns the reason for what was given, at "but if he hears you, you have gained your brother." Thus I have said that the little ones are not to be despised, but what is to be done if someone gives scandal? Here he teaches. "But if your brother sins against you, go and rebuke him between you and him alone." Note first that he says "has sinned": hence he speaks of a sin already perpetrated. Hence one must proceed differently with a sin already perpetrated than with one being perpetrated, because what has been perpetrated cannot become unperpetrated; hence while it is being perpetrated, one must take pains that it not happen; Isaiah 58:6: "undo the bonds of wickedness, loose the bundles that weigh down," etc. Hence while it is being perpetrated it is not necessary that such an order be kept; when it has been perpetrated, it is necessary. Again he says "against you." The Gloss: if he has inflicted an injury or an insult on you. Hence he means to say that we should forgive a sin done against us; but a sin that is done against God we cannot forgive, as the Gloss says. 1 Kings 2:25: "if a man sins against God, who shall pray for him?" Again, you ought especially to attend to injuries done by one who is with you in the same fellowship; of others too care must be had, but not so much. 1 Corinthians 5:12: "what have we to do with judging those who are outside?" "Go and rebuke him between you and him alone." The Lord leads the disciples to perfect solicitude and correction. Above, at 5:23, the Lord had said that if someone offended his brother, he should leave his gift before the altar, etc.; but here he goes further, because it is not only the one who injured, but the one who was injured: hence, "if he sins against you, go," etc.; Psalm 119:7: "with those who hated peace, I was peaceable." And should you forgive first? No; rather you ought first to rebuke: hence he does not command forgiving anyone, but the one who repents. Again he says "rebuke," not "upbraid" or "exasperate," and show it briefly. If he acknowledges it, then you ought to forgive; hence, "instruct them in a spirit of gentleness," Galatians 6:1. But does the one who omits this correction sin? Augustine: if you do not rebuke, you have become worse by keeping silent than he by sinning. But since it is true that all are bound to rebuke, someone might say that it belongs to prelates alone by office, but to others by charity. Sometimes the Lord permits the good to be punished along with the wicked. Why? Because they did not rebuke the wicked. Yet Augustine says that sometimes we ought to desist, if you fear that on account of this correction they will not be amended, but rendered worse. Again, if you fear it may bring on persecution of the Church, you do not sin. But if you desist so as not to be harmed in temporal things, so that no trouble befall you, or the like, you sin; Proverbs 9:8: "reprove a wise man, and he will love you." "Rebuke him between you and him alone." And why? Because this correction is done out of charity; and charity is the love of God and of neighbor. If you love, you ought to cherish his salvation. But in this two things must be attended to, namely conscience and good repute. If therefore you wish to save him, you ought to save his reputation; and this you will do by correcting him between you and him. If you rebuke him before everyone, you take away his reputation; yet conscience is to be preferred to reputation. Yet it frequently happens that when a man sees his sin made public, he becomes so shameless that he exposes himself to every sin; Jeremiah 2:20: "under every leafy tree you lay down, harlot"; Ecclesiasticus 4:25: "there is a shame that brings sin." But against this an objection is raised from 1 Timothy 5:20: "rebuke him that sins, before all." And this is true if the offense is public. For there is someone who sins publicly, and then he must be rebuked publicly; and someone who sins secretly, and then he must be rebuked secretly; and this is clear, because Augustine says that if he has sinned with you alone knowing it, "rebuke him between you and him alone." "But if he hears you, you have gained your brother." To what end does he say this? On account of three things. That you may know for what end you ought to rebuke: because if you rebuke for your own sake, you accomplish nothing, since where the amends sought is private, the correction is not meritorious; but if for God's sake, then it avails. Again, this is what you ought to intend, namely to instill the rebuke and teaching into the mind of the brother. Again, someone might say that it would not be just to lose one's brother. But if that were so, he would not have said, "you have gained your brother." Again, "gained," because he is a fellow-member of yours: and just as a member has fellow-feeling with a member, so too you with your brother. Again, "gained," because you gain salvation for yourself; James 4:11: "he who judges his brother detracts from the law, and judges the law"; hence in the same epistle, 5:20: "he who causes a sinner to be converted from the error of his way saves his soul from death, and covers a multitude of sins." "But if he does not hear you, take with you one or two." Here he brings in the testimony, "take one or two," etc. Deuteronomy 19:15: "in the mouth of two or three witnesses let every word stand." But here is a question: why does he not bring in witnesses right away? It must be said that conscience must be cleansed in such a way that reputation is not harmed: hence if he can correct him first and by himself, well and good; if not, then let him call in witnesses. And Jerome says that first one, and afterward two. And why? That they may be witnesses of the correction made, because if he proceeds further, it is not to be imputed to you. Jerome says it is also for another purpose, namely so that he may be convicted of the sin: because some are so obstinate that they do not acknowledge it, and therefore you ought to bring witnesses, so as to convict him of the deed. Or perhaps he will repeat the injury. Or, according to Augustine, to convict him. But against this seems to be what Augustine says, that before showing it to two, he ought to show it to the superior, and this is to show it to the Church. Therefore it seems to invert the order. I say that it can be shown to the prelate either in a judicial order, or as to a private person. Augustine means, therefore, that it ought to be shown to the superior first as to a private person, so that, as a private person, he may take pains toward the correction. Hence he says, "if he does not hear them, tell it to the Church." Here the denunciation is set forth. And first he denounces; second the sentence is set forth; third the efficacy. The second at "but if he does not hear the Church," etc.; the third at "Amen I say to you," etc. He says, "but if he does not hear them, tell it to the Church," that is, to the whole multitude, that he may be confounded, so that he who was unwilling to be rebuked without confusion may be rebuked with confusion. For there is a shame that brings sin, and there is a shame that brings glory and grace, Ecclesiasticus 4:25. Or "to the Church," that is, to the judges, that he may be corrected; Deuteronomy 21:18: "if a man has a stubborn and defiant son who will not hear the command of his father and mother, and, having been disciplined, refuses to obey, they shall seize him and bring him to the elders of that city, and to the gate of judgment," etc. Then the penalty is added: "if he does not hear the Church, let him be to you as a heathen and a publican." Heathens are the gentiles and unbelievers; publicans are those who collect the tribute, who are public sinners. Hence, as if set apart, let them be excommunicated by the sentence of the Church, because they did not hear the Church. Hence for contumacy alone a man can be excommunicated. "Amen I say to you," etc. Here the efficacy of this sentence is set forth. For someone might say: what do I care if it is told to the Church, and I am excommunicated? Therefore he shows this efficacy: "Amen I say to you, whatever you shall bind on earth shall be bound also in heaven; and whatever you shall loose on earth shall be loosed also in heaven." Above, these things were said to Peter; but here it is said to the whole Church. And he is said to bind, either because he does not absolve, or because he excommunicates. Origen says that here he says "in heaven," whereas when he spoke to Peter, he said "in the heavens," to designate that Peter has universal power. But here he says "in heaven," because their power is not universal, but limited to some place, since he gave universal power to Peter. "Again I say to you," etc. Here he sets forth the efficacy of prayer. And first he does this; second he gives the reason, at "for where there are two," etc. He says, "again I say to you," etc. But you can object, because we ask for many things which we do not obtain. This happens, first, on account of the unworthiness of those asking; hence he says "two of you," namely, who live according to the Gospel. James 4:3: "you ask, and do not receive, because you ask wrongly." Again, it happens because they do not agree together, because they do not have the bond of peace: for it is impossible that the prayers of many not be heard, if out of many prayers there is made, as it were, one; 2 Corinthians 1:11: "so that, from the persons of the many faces, thanks may be given by many on our behalf for the gift that is in us." Again, because they ask for certain things which are not expedient for them unto salvation: for the petition ought to be about something useful; below, 20:22: "you do not know what you ask." "It shall be done for them by my Father," who is in heaven, that is, in the heights: or "in heaven," that is, in us. "For where there are two or three gathered together in my name, there am I in the midst of them"; in a gathering of the holy, not of the earthly. Psalm 110:1: "great are the works of the Lord, in the council of the just and in the congregation." "Where therefore two or three." Charity is not in one, but in many; hence 1 John 4:16: "he who abides in charity abides in God, and God in him." Therefore, "I am in the midst of them." "Then Peter, coming to him, said: Lord, how often shall my brother sin against me, and I forgive him?" Above he taught in what order forgiveness is to be given, namely after correction and amends; here he deals with the number of times forgiveness is to be given. First, therefore, Peter's question is set forth; second, Christ's response; third, a comparison is applied. The second at "Jesus says to him," etc.; the third at "the kingdom of heaven is likened." He says, therefore, "then, coming." "Then," namely, having heard this saying, "if your brother sins against you," etc., Peter was then moved to ask whether he should forgive once or many times, and he said, "how often shall my brother sin against me," etc., "shall I forgive up to seven times," as if to say: up to seven times is a matter of weakness, but more is a matter of malice. Therefore he asked whether he should forgive up to seven times. Again, he knew that which is said in 4 Kings 5:10, that Elisha commanded Naaman to wash himself seven times in the Jordan; therefore he thought that he ought to forgive seven times. Jesus says to him: "I do not say to you seven times, but seventy times seven." In one way this can be taken, what he says "seven times," cumulatively, so that the sense is: not seven times, but seventy times. Or it can be taken multiplicatively, so that seventy times seven; and thus Jerome expounds it. According to the first exposition, which is Augustine's, it is given to be understood that we ought to forgive entirely, just as Christ forgave everything. Colossians 3:13: "forgiving one another, if anyone has a complaint against another; even as the Lord has forgiven you, so also do you." Or it can be said that a finite number is put for an infinite one, as in the Psalms: "the word which he commanded unto a thousand generations." According to Jerome, the cause is the same; yet a reason for the number is added. For by six perfection is signified; by the hundred, which is multiplied by ten, the Decalogue is signified. The first number that departs from ten is the eleventh. And because by six the totality is signified, therefore the totality of sins is signified; as if he were to say: whatever things your brother has sinned against you, forgive him. Hence, according to Jerome, it seems that he means to say that one can forgive more than the brother could possibly offend.”

Post-Reformation

c. 1650 – 1900
1771
A.D.
John Gill Reformed
1697–1771
“Even so it is not the will of your father which is in heaven,.... This is the accommodation, or application of the parable of the lost sheep to the present purpose, and is the top of the climax or gradation here made use of. First, Christ observes, in order to deter any from despising and offending any of his disciples, even the meanest, that they have angels to be their guardians, who are continually in the presence of God; and next, that he himself in human nature came to be the author of salvation to these persons; and then rises up to the sovereign will of his Father, and their's, the source and security of their everlasting happiness; which will is, not that one of these little ones, that believe in Christ, should perish. It is his will of command that no stumbling block should be laid in their way to cause them to stumble and fall, to the grieving of their souls, the wounding of their consciences, and the perishing, or loss of their peace and comfort; and it is his unalterable will of purpose, or his unchangeable decree, that not one of them, even the meanest, shall perish eternally: in pursuance of which will, he has chosen them in his Son, he has put them into his hands, and secured them in his covenant; and having redeemed them by Christ, and called them by grace, he keeps them by his power, through faith unto salvation. Nor shall anyone of them finally and totally fall away and perish, through the power of their own corruptions, the temptations of Satan, the reproaches and persecutions of men, the frowns or flatteries of the world, or through the errors and heresies of false teachers, or any other way. It is to be observed, that when our Lord, in Mat 18:10, is speaking of the happiness of the angels, and the honour done to the little ones by having such guardians; then the more to aggrandize this matter, he represents those as in the presence of his "Father which is in heaven"; but here, when he would express the wonderful love and grace of God, in the resolutions of his heart, and purposes of his will, to save them, then it is "your Father which is in heaven"; and this, the rather to engage them to the belief of it, since they stood in such a near relation to him, as children to a father: and therefore must be infinitely more concerned for their welfare, than a proprietor of sheep can be, for one that is lost. The Arabic and Ethiopic versions indeed read, "my father", but without any authority; for the phraseology, "the will before your Father", as in the original text; see Gill on Mat 11:26”
Source
1849
A.D.
1774–1849
“If a man have a hundred sheep. This is to shew the goodness and mercy of God towards sinners. By the one sheep, some understand all mankind, and by the ninety-nine, the angels in heaven. (Witham) — Jesus Christ manifests his tender regard and solicitude for us poor weak creatures, by becoming himself the Son of man, thus abandoning in some measure the angels who are in heaven. He is come down upon earth to save by his death what was lost, imitating thus, with regard to men, the conduct themselves observe with regard to their sheep. (Bible de Vence) — In the Greek, it is dubious whether the shepherd leaves the ninety-nine in the mountains, or, whether he himself goeth into the mountains in quest of the lost sheep.”
Source
1871
A.D.
1871
“How think ye? If a man have an hundred sheep, and one of them be gone astray, &c.--This is another of those pregnant sayings which our Lord uttered more than once. See on the delightful parable of the lost sheep in Luk 15:4-7. Only the object there is to show what the good Shepherd will do, when even one of His sheep is lost, to find it; here the object is to show, when found, how reluctant He is to lose it. Accordingly, it is added,”
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.