2 Corinthians 5:21 "God made him who had no sin to be sin for us, so that in him we might become the righteousness of God."
The Bible teaches that Christ became sin for us. But this always seemed somewhat arbitrary to me. It does not seem just for God to place the burden of my sin upon someone else. What about the sins I have committed against others? Doesn't the blame now fall upon God? How can they forgive him?
I believe that it was not by an arbitrary act that God just as soon could have propitiated by means of a gnat or a rock that Jesus became our sin, because he would have been abandoning justice in the process and forcing the victims of sin to now hold him accountable with no avenue of reconciliation. However, in Christ's fullness as God, by giving us mercy he took upon himself the responsibility for the sin he had forgiven and then he bore the punishment. In doing so he is was able forgive me of my sin, but also to be forgiven of my sin.
This is how he "became sin" and how he is both just and the justifier of those who have faith in Jesus.
Romans 3:26 "he did it to demonstrate his righteousness at the present time, so as to be just and the one who justifies those who have faith in Jesus."
Here is a related post "I Forgive God of His Sins Against Me".