What does 2 Corinthians 5:21 mean?

Paul puts even more clearly the central message of the gospel. This is the same teaching he had delivered to the Corinthians and to many other people around the world. This verse may be the most concise presentation of the gospel in all of Scripture.

God acted for our sake. That means God acted out of His love, to make it possible to remove the separation between us and Him: our sin. To accomplish this, God made Christ, who had never sinned during His life on earth in any way, to become our sin. Jesus' death, then, paid the price for our sin, removing our guilt and removing the obstacle between us and God. Instead of "being sin" ourselves, those who come to God through faith in Christ are given credit for Christ's righteous, sinless life. We "become God's righteousness" and are reconciled in our relationship with Him.

In short, by His gift of grace and through our faith in Christ (Ephesians 2:8–9), God receives Christ's death as payment for our sin and gives us credit for Christ's righteousness in return. That's what it means to be "in Christ."