Aquinas:

[v]18 All this is from God, who through Christ reconciled us to himself and gave us the ministry of reconciliation; 19 that is, in Christ God was reconciling the world to himself, not counting their trespasses against them, and entrusting to us the message of reconciliation. 20 So we are ambassadors for Christ, God making his appeal through us. We beseech you on behalf of Christ, be reconciled to God. 21 For our sake he made him to be sin who knew no sin, so that in him we might become the righteousness of God.

194. – After discussing the saints’ reward and how they prepared themselves to receive it, the Apostle now treats of the cause of both and does three things. First, he shows that the Author of all these things is God; secondly, he recalls the benefit conferred by Christ (v. 18b); thirdly, the use of the benefit (v. 20)

195. – He says therefore: I have said that we intend the salvation of our neighbor and that the old things have passed away; but all this is from God the Father, or from God as author: “For from him and through him and to him are all things” (Rom. 11:36); “Every good endowment and every perfect gift is from above, coming down from the Father of lights” (Jas. 1:17).

196. – Then he mentions the benefits received from God (v. 18): first, he mentions the benefit received; secondly, he explains it (v. 19).

197. – He recalls two benefits conferred by Christ: one is common and the other is special. Common to the whole world was reconciliation to God. And this is what he says, all this is from God, namely, God the Father, who reconciled us to God, i.e., made peace between us and God. And this is by Christ, i.e., by the Incarnate Word. For men were enemies of God because of sin, but Christ removed this enmity from their midst, satisfying for sin and producing harmony: “Whether on earth or in heaven, making peace by the blood of his cross” (Col. 1:20). Therefore he says, through Christ: “We were reconciled to God by the death of his Son” (Rom. 5:10). But a special gift was conferred on the apostles, namely, that they are ministers of this reconciliation. Hence he says, and gave us, the apostles and vicars of Christ, the ministry of reconciliation: “Who has made us competent to be ministers of a new covenant” (2 Cor. 3:6); “Let the mountains,” i.e., the apostles, “bear prosperity for the people” (Ps. 72:3), namely, from the Lord.

198. – Then when he says, that is, he explains what he has said. First, the first thing; secondly, the second (v. 19b). He says therefore: I say that God reconciled us to himself in this way. For there were enmities between God and man on account of sin, as has been said: “But your iniquities have made a separation between you and your God” (Is. 59:2). Therefore, sin being destroyed by the death of Christ, the enmities were dissolved. And this is what he says: That is, in Christ God by oneness of essence: “I am in the Father and the Father in me” (Jn. 14:11). Or in Christ God was reconciling the world to himself through Christ: “We were reconciled to God by the death of his Son” (Rom. 5:10). This he did, not counting their trespasses against them, i.e., not retaining in his memory their sins, actual or original, to punish them, for which Christ fully satisfies. According to this he is said to have reconciled us to himself, inasmuch as he does not impute our sins to us: “Blessed is the man to whom the Lord imputes no iniquity (Ps. 32:2). Then when he says, and entrusting to us the message of reconciliation, he explains the second thing, namely, the benefit conferred on the apostles. As if to say: he has given us the mystery of reconciliation in this way, namely, that he has entrusted to us the message of reconciliation, i.e., he has given the power and has inspired in our hearts to announce to the world that this reconciliation was made by Christ. By doing this we induce men to conform themselves to Christ by baptism: “Behold, I have put my words in your mouth” (Jer. 1:9).

199. – Then when he says, So we are ambassadors for Christ, he indicates the use of the benefit. First, as to the second benefit conferred on the apostles; secondly, as to the first benefit conferred on all (v. 20b). He says therefore: since God has established the word of reconciliation, we ought to use it. This is why we are ambassadors for Christ: “For which I am an ambassador in chains; that I may declare it boldly, as I ought to speak” (Eph. 6:20). Our fitness for this ambassadorship is from God’s power, which is in me. Hence he says, God making his appeal through us, because God, who speaks in us, makes us fit for this ambassadorship: “For it is not you who speak, but the Spirit of your Father speaking through you” (Matt. 10:20); “Since you desire proof that Christ is speaking in me” (2 Cor. 13:3).

200. – The when he says, we beseech you, he describes the use of the first benefit. First, he exhorts to its use; secondly, he shows the source of his power to reconcile to God (v. 21). He says therefore: inasmuch as God has produced a reconciliation and we are ambassadors of God in this, we beseech you on behalf of Christ. He speaks gently, even though he could have commanded: “Convince, rebuke, and exhort, be unfailing in patience and in teaching” (2 Tim. 4:2); “Accordingly, though I am bold enough in Christ to command you to do what is required, yet for love’s sake I prefer to appeal to you” (Phm. 1:8-9). We beseech you, I say, on behalf of Christ, i.e., for the love of Christ, to be reconciled to God. But this seems contrary to his statement that God has reconciled us to himself. Therefore, if he reconciled us, what need is there to be reconciled? For we are already reconciled. I answer that God reconciled us to himself as efficient cause, namely, on his part, but in order that it be meritorious for us, it is necessary that reconciliation be made on our part, namely, in baptism and in penance. And then we cease from sins.

201. – Where we get the faculty to reconcile to God is indicated by the fact that he gave us the power to live justly and abstain from sins. By doing this we are reconciled to God. Hence he says, for our sake, he made him to be sin who knew no sin. As if to say: you can be reconciled to God, because he, namely, Christ, who knew no sin: “He committed no sin; no guile was found on his lips” (1 Pet. 2:22); “Which of you convicts me of sin?” (Jn. 8:46). For our sake, he made him to be sin. This can be explained in three ways. In one way because it was the custom of the Old Law to call a sacrifice for sin “sin”: “They feed on the sin of my people” (Hos. 4:8), i.e., the offerings for sin. Then the sense is: he made him to be sin, i.e., the victim of sacrifice for sin. In another way, because sin is sometimes taken for the likeness of sin, or the punishment of sin: “God sending his own Son in the likeness of sinful flesh and for sin, he condemned sin in the flesh” (Rom. 8:3). Then the sense is: he made him to be sin, i.e., made him assume mortal and suffering flesh. In a third way, because one thing is said to be this or that, not because it is so, but because man considers it such. Then the sense is: he made him to be sin, i.e., made him regarded a sinner: “He was numbered with the transgressors” (Is. 53:12).

202. – He did this, so that in him we might become the righteousness [justice] of God, i.e., justified by God. Or justice, because he not only justified us, but also willed that others be justified by us. The justice, I say, of God, not ours. And in Christ, i.e., through Christ. Or another way, that Christ himself be called justice. Then the sense is this: that we might become the righteousness [justice], i.e., cling to Christ by love and faith, because Christ is justice itself. But he says, of God, to exclude man’s justice, by which a man trusts in his own merits: “For, being ignorant of the righteousness that comes from God, and seeking to establish their own, they did not submit to God’s righteousness” (Rom. 10:3). In him, namely, in Christ, i.e., by Christ, because he was made justice for us (1 Cor. 1:30).

Thomas Aquinas, Commentary On the First Epistle to the Corinthians, trans., Fabian Larcher; http://dhspriory.org/thomas/SS2Cor.htm#55 (accessed 10 August 2011).

Credit to Michael for the find.