Psalm 130:3-4 (NIV)
If you, O Lord, kept a record of sins, O Lord, who could stand? But with you there is forgiveness; therefore you are feared.
Sin separates us from God who is of holier eyes than to behold evil. God must punish sin; He already has, once for all. His wrath and judgment upon sin was carried out upon Christ on the cross.
1 Peter 3:18 (NKJV)
For Christ also suffered once for sins, the just for the unjust, that He might bring us to God, being put to death in the flesh but made alive by the Spirit.
Hebrews 9:28a (NKJV)
Christ was offered once to bear the sins of many.
God is a God of justice but he is also a God of love and mercy. He has satisfied his justice and now can rightly offer forgiveness. The Psalm above says that when God forgives, he forgets. He does not keep a record of forgiven sins. But perhaps you and I should never forget so that we can remember all that God has done!