The Enchiridion by St. Augustine
Chapter 41.--Christ, Who Was Himself Free from Sin, Was Made Sin for Us, that We Might Be Reconciled to God.
Begotten and conceived, then, without any indulgence of carnal lust, and therefore bringing with Him no original sin, and by the grace of God joined and united in a wonderful and unspeakable way in one person with the Word, the Only-begotten of the Father, a son by nature, not by grace, and therefore having no sin of His own; nevertheless, on account of the likeness of sinful flesh in which He came, He was called sin, that He might be sacrificed to wash away sin. For, under the Old Covenant, sacrifices for sin were called sins. And He, of whom all these sacrifices were types and shadows, was Himself truly made sin. Hence the apostle, after saying, |We pray you in Christ's stead, be ye reconciled to God,| forthwith adds: |for He hath made Him to be sin for us who knew no sin; that we might be made the righteousness of God in Him.| He does not say, as some incorrect copies read, |He who knew no sin did sin for us,| as if Christ had Himself sinned for our sakes; but he says, |Him who knew no sin,| that is, Christ, God, to whom we are to be reconciled, |hath made to be sin for us,| that is, hath made Him a sacrifice for our sins, by which we might be reconciled to God. He, then, being made sin, just as we are made righteousness (our righteousness being not our own, but God's, not in ourselves, but in Him); He being made sin, not His own, but ours, not in Himself, but in us, showed, by the likeness of sinful flesh in which He was crucified, that though sin was not in Him, yet that in a certain sense He died to sin, by dying in the flesh which was the likeness of sin; and that although He Himself had never lived the old life of sin, yet by His resurrection He typified our new life springing up out of the old death in sin.