John Trapp on 2 Corinthians 5:21

Verſe 21. To be ſin for us] That is, a ſin-offering, or an exceeding ſinner, as Exod. 29.14. So Chriſt was, 1. By imputation, for our ſins were made to meet upon him, as that Evangelicall Prophet hath it, See Aug. Enchirid. c. 41. Iſa. 53 6. And ſecondly, By reputation, for he was reckoned among malefactours, ibid. And yet one Auguſtinus de Roma Archbiſhop of Nazareth was cenſured in the Councel of Baſil, for affirming that Chriſt was peccatorum maximus, the greateſt of ſinners. Chriſt ſo loved us, ſaith one, that he endured that which he moſt hated, to become ſin for us (he was made ſin paſſive in himſelf to ſatisfie for ſin active in us) and the want of that which was more worth then a world to him, the ſenſe of Gods favour for a time, Ama amorem illius, &c. ſaith Bernard.

Who knew no ſinne] That is, With a practicall knowledge, with an intellectuall he did, elſe he could not have reproved it. We know no more then we practiſe. Chriſt is ſaid to know no ſin, becauſe he did none.

That we might be made, &c.] As Chriſt became ſin, not by ſin inherent in him, but by our ſin imputed to him; ſo are we made the righteouſneſſe of God, by Chriſts righteouſneſſe imputed and given unto us. This the Papiſts jearingly call putative righteouſneſſe.