Prolly owing to the increasing influence of the theological errors of Luther that, like cancer, have been spreading throughout the Body of Christ, one hears more and more preachers (even Prelates) stating that Christ became sin but that is an obvious absurdity - He who is without sin, Jesus Christ, can not become sin anymore than a man can become a woman - but because Catholics have been deprived of exegetical tradition, they are vulnerable to such an ancient and execrable lie.
When in doubt over exegesis, it always help to go back to a great source;
The Great Commentary of Cornelius a Lapide:
Ver. 21.—Him who knew no sin. Experimentally, says S. Thomas, Christ knew no sin, though by simple knowledge He did, for He did no sin.
Hath made Him to be sin for us.
For us, says Illyricus, who were sin; because, he says, sin is the substance and form of our soul. But to say this of ourselves is folly, of Christ blasphemy. (1.) The meaning is that God made Christ to be the victim offered for our sin, to prevent us from atoning for our sins by eternal death and fire. The Apostle plays on the word sin, for when he says, “Him who knew no sin,” he means sin strictly speaking; but when he says, “He made Him to be sin for us,” he employs a metonymy. So Ambrose, Theophylact, and Anselm. In Ps. xl. 12, Christ calls our sins His. (2.) Sin here denotes, says S. Thomas, the likeness of sinful flesh which He took, that He might be passible, just as sinners who are descended from Adam are liable to suffering. (3.) Sin, in the sense of being regarded by men as a noteworthy sinner, and being crucified as a malefactor. So the Greek Fathers.
When in doubt over exegesis, it always help to go back to a great source;
The Great Commentary of Cornelius a Lapide:
Ver. 21.—Him who knew no sin. Experimentally, says S. Thomas, Christ knew no sin, though by simple knowledge He did, for He did no sin.
Hath made Him to be sin for us.
For us, says Illyricus, who were sin; because, he says, sin is the substance and form of our soul. But to say this of ourselves is folly, of Christ blasphemy. (1.) The meaning is that God made Christ to be the victim offered for our sin, to prevent us from atoning for our sins by eternal death and fire. The Apostle plays on the word sin, for when he says, “Him who knew no sin,” he means sin strictly speaking; but when he says, “He made Him to be sin for us,” he employs a metonymy. So Ambrose, Theophylact, and Anselm. In Ps. xl. 12, Christ calls our sins His. (2.) Sin here denotes, says S. Thomas, the likeness of sinful flesh which He took, that He might be passible, just as sinners who are descended from Adam are liable to suffering. (3.) Sin, in the sense of being regarded by men as a noteworthy sinner, and being crucified as a malefactor. So the Greek Fathers.
Of these three interpretations the first is the more full, significant, and vigorous, and the one more consonant with the usage of Scripture, which frequently speaks of an expiatory victim as sin. Cf. Hosea iv. 8; Lev. iv. 24 and 21; Ezek. xliv. 29. The reason of this metonymy is that all the punishment and guilt of the sin were transferred to the expiatory victim, and so the sin itself might seem to be also transferred to it. In token of this the priest was accustomed to lay his hands on the victim, and call down on it the sins of the people; for by the hands are signified sinful actions, which are for the most part executed by the hands, as Theodoret says in his notes on Leviticus i.
Therefore the laying of hands on the victim was both a symbol of oblation and a testimony of the transference of guilt to the victim, showing that it was expiatory, and that it bore the sin itself, with all its burden of guilt and punishment. In this way the high-priest on the great Day of Atonement turned a goat into the wilderness, having imprecated on it the sins of the whole people. Cf. Lev. xvi. 20.
That we might be made the righteousness of God in Him.
Heretics raise the objection that Christ was made for us sin, in the sense that our sin was imputed to Him and was punished in Him; therefore we are made the righteousness of God, because it is imputed to us. I answer that the two things are not parallel; for Christ could not really be a sinner as we can really be righteous, nor does the Apostle press the analogy. He only says that Christ bore our sins, that we through Him might be justified. Moreover, Christ actually was made sin, i.e., a victim for sin (this is the meaning of “sin” here), and therefore we truly become the righteousness of God. So easily and completely can we turn the tables on these Protestant objectors.