"Remember not the sins of my youth, nor my transgressions: according to thy mercy remember thou me for thy goodness' sake, O Lord." -Psalm 25:7, KJV.

God is so merciful to us, and one of the main sources of His mercy is letting us serve Him in spite of our past. God has forgiven us of all we've done, and as a result, we can still serve Him, living a successful Christian life. What a wonderful God we serve!