That is the mystery which is rich in divine grace to sinners:  wherein by a wonderful exchange our sins are no longer ours but Christ’s and the righteousness of Christ is not Christ’s but ours.

From 2 Corinthians 5:20-21:  “So we are ambassadors for Christ, since God is making His appeal through us; we entreat you on behalf of Christ, be reconciled to God.  For our sake He made Him to be sin who knew no sin, so that in Him we might become the righteousness of God.”