A - Thanks for the question. Here is the verse you are referring to.
"For our sake he made him to be sin who did not know sin, so that we might become the righteousness of God in him." - 2 Cor 5:21What Paul is doing in this verse is setting up a paradox. We are the sinners. God alone is righteous. So, for us to share in God's righteousness, Christ shares in our sinful nature. Not sinning himself, but rather, taking on our fallen nature and redeeming it by his sinlessness.
This statement is basically a restatement of other verses such as:
For what the law, weakened by the flesh, was powerless to do, this God has done: by sending his own Son in the likeness of sinful flesh and for the sake of sin, he condemned sin in the flesh, - Rom 8:3This is the Gospel. We have been redeemed by Christ's suffering. He emptied himself, taking the form of a slave in order to ransom us from death and sin and offer us life eternal. This was done by Christ's passion, death and resurrection and our joining him in faith and baptism.
Christ ransomed us from the curse of the law by becoming a curse for us, for it is written, "Cursed be everyone who hangs on a tree," - Gal 3:13
He himself bore our sins in his body upon the cross, so that, free from sin, we might live for righteousness. By his wounds you have been healed. - 1 Pet 2:24
If we get more technical as to why Paul says "made sin" there are different interpretations, but all reach basically the same conclusion which is summarized above - that Christ became an offering for our sins.
In the Greek Old Testament "sin" is shorthand expression for a Levitical priestly "sin offering". Hence, we see this in:
This bullock must also be brought outside the camp and burned, just as has been prescribed for the other one. This is the sin offering for the community. - Lev 4:21We also see it in the suffering servant of Isaiah:
(But the LORD was pleased to crush him in infirmity.) If he gives his life as an offering for sin, he shall see his descendants in a long life, and the will of the LORD shall be accomplished through him. - Is 53:10God reconciles us to himself through Christ's offering.
The good news in one verse.