Objection 1. It would seem that there was sin in Christ. For it is written (Psalm 21:2): "O God, My God... why hast Thou forsaken Me? Far from My salvation are the words of My sins." Now these words are said in the person of Christ Himself, as appears from His having uttered them on the cross. Therefore it would seem that in Christ there were sins.
Objection 2. Further, the Apostle says (Romans 5:12) that "in Adam all have sinned"--namely, because all were in Adam by origin. Now Christ also was in Adam by origin. Therefore He sinned in him.
Objection 3. Further, the Apostle says (Hebrews 2:18) that "in that, wherein He Himself hath suffered and been tempted, He is able to succor them also that are tempted." Now above all do we require His help against sin. Therefore it seems that there was sin in Him.
Objection 4. Further, it is written (2 Corinthians 5:21) that "Him that knew no sin" (i.e. Christ), "for us" God "hath made sin." But that really is, which has been made by God. Therefore there was really sin in Christ.
Objection 5. Further, as Augustine says (De Agone Christ. xi), "in the man Christ the Son of God gave Himself to us as a pattern of living." Now man needs a pattern not merely of right living, but also of repentance for sin. Therefore it seems that in Christ there ought to have been sin, that He might repent of His sin, and thus afford us a pattern of repentance.
On the contrary, He Himself says (John 8:46): "Which of you shall convince Me of sin?"
I answer that, As was said above (Question 14, Article 1), Christ assumed our defects that He might satisfy for us, that He might prove the truth of His human nature, and that He might become an example of virtue to us. Now it is plain that by reason of these three things He ought not to have assumed the defect of sin. First, because sin nowise works our satisfaction; rather, it impedes the power of satisfying, since, as it is written (Sirach 34:23), "The Most High approveth not the gifts of the wicked." Secondly, the truth of His human nature is not proved by sin, since sin does not belong to human nature, whereof God is the cause; but rather has been sown in it against its nature by the devil, as Damascene says (De Fide Orth. iii, 20). Thirdly, because by sinning He could afford no example of virtue, since sin is opposed to virtue. Hence Christ nowise assumed the defect of sin--either original or actual--according to what is written (1 Peter 2:22): "Who did no sin, neither was guile found in His mouth."
Reply to Objection 1. As Damascene says (De Fide Orth. iii, 25), things are said of Christ, first, with reference to His natural and hypostatic property, as when it is said that God became man, and that He suffered for us; secondly, with reference to His personal and relative property, when things are said of Him in our person which nowise belong to Him of Himself. Hence, in the seven rules of Tichonius which Augustine quotes in De Doctr. Christ. iii, 31, the first regards "Our Lord and His Body," since "Christ and His Church are taken as one person." And thus Christ, speaking in the person of His members, says (Psalm 21:2): "The words of My sins"--not that there were any sins in the Head.
Reply to Objection 2. As Augustine says (Gen. ad lit. x, 20), Christ was in Adam and the other fathers not altogether as we were. For we were in Adam as regards both seminal virtue and bodily substance, since, as he goes on to say: "As in the seed there is a visible bulk and an invisible virtue, both have come from Adam. Now Christ took the visible substance of His flesh from the Virgin's flesh; but the virtue of His conception did not spring from the seed of man, but far otherwise--from on high." Hence He was not in Adam according to seminal virtue, but only according to bodily substance. And therefore Christ did not receive human nature from Adam actively, but only materially--and from the Holy Ghost actively; even as Adam received his body materially from the slime of the earth--actively from God. And thus Christ did not sin in Adam, in whom He was only as regards His matter.
Reply to Objection 3. In His temptation and passion Christ has succored us by satisfying for us. Now sin does not further satisfaction, but hinders it, as has been said. Hence, it behooved Him not to have sin, but to be wholly free from sin; otherwise the punishment He bore would have been due to Him for His own sin.
Reply to Objection 4. God "made Christ sin"--not, indeed, in such sort that He had sin, but that He made Him a sacrifice for sin: even as it is written (Hosea 4:8): "They shall eat the sins of My people"--they, i.e. the priests, who by the law ate the sacrifices offered for sin. And in that way it is written (Isaiah 53:6) that "the Lord hath laid on Him the iniquity of us all" (i.e. He gave Him up to be a victim for the sins of all men); or "He made Him sin" (i.e. made Him to have "the likeness of sinful flesh"), as is written (Romans 8:3), and this on account of the passible and mortal body He assumed.
Reply to Objection 5. A penitent can give a praiseworthy example, not by having sinned, but by freely bearing the punishment of sin. And hence Christ set the highest example to penitents, since He willingly bore the punishment, not of His own sin, but of the sins of others.
Source: Summa Theologica (New Advent)