In 2 Corinthians 5:17-21, Paul speaks of what has come to be known as the “great exchange”—our sins are imputed to Christ and Christ’s righteousness is imputed to us. According to Paul

If anyone is in Christ, he is a new creation; the old has gone, the new has come! All this is from God, who reconciled us to himself through Christ and gave us the ministry of reconciliation: that God was reconciling the world to himself in Christ, not counting men's sins against them. And he has committed to us the message of reconciliation. We are therefore Christ's ambassadors, as though God were making his appeal through us. We implore you on Christ's behalf: Be reconciled to God. God made him who had no sin to be sin for us, so that in him we might become the righteousness of God.

For Paul, to be in Christ is to participate even now in the new creation which dawns when God raised Jesus from the dead. Paul is crystal clear that the reconciliation of God to the sinner and the sinner to God is accomplished by Christ, for us, on our behalf. “God was in Christ, reconciling the world to himself.” This is not something we can do through our own pitiful efforts to appease God or satisfy his justice. Our reconciliation comes about only because Christ has been made sin (through imputation) so that we become righteous (likewise, through imputation). This is exactly the same thing Paul teaches in Romans 5:18-19:

Consequently, just as the result of one trespass was condemnation for all men, so also the result of one act of righteousness was justification that brings life for all men. For just as through the disobedience of the one man the many were made sinners, so also through the obedience of the one man the many will be made righteous.

~ Kim Riddlebarger