This is a fairly common question and the Bible provides a good answer. Allow me to explain.
When a person recognizes their sinful condition and comes by faith to Jesus, accepting the sacrifice He made on the cross and receiving Him as their Savior, that person is born again and completely forgiven of all their sins — past, present and future. This forgiveness is FOR SALVATION and it is perpetual. (1 John 1:7)
Why, then, does the Bible tell us to come to the Lord and say, "Forgive us our trespasses..." if we are already forgiven by the cross of Christ? The reason is for RELATIONSHIP.
If you are a Christian, a believer and follower of Christ, you are in a relationship with God through the Son, and that relationship can easily become strained and sullied by living in a world where sin is commonplace. This creates the need for us to come to the cross and find forgiveness so that no barriers exist between us and our Lord.
I’ll use the analogy of marriage to explain our relationship with God and why we need to come to Him and ask forgiveness. My wife and I are married and our status as a married couple is perpetual, but because we have a close relationship we sometimes do or say things that hurt the other person. When we do, we remain married, but there now exists a breach or rift in our relationship that needs to be mended before we can move on and be close once again. So, we come together and confess our wrong attitudes, words and actions, and we extend forgiveness where it is needed. In this way, the closeness of our relationship is restored through forgiveness. But throughout this process our status as a married couple never changes.
In the same way, your relationship with God can easily become strained and distant through sin. It doesn't mean you've lost your salvation or are no longer a child of God — that status cannot be affected by sin. But it DOES affect your relationship with God, and that relationship (closeness) needs to be restored. This is why we come to the Lord and confess our sins — so that nothing is able to create distance between us and our Redeemer.
This picture of cleansing is powerfully depicted in the foot washing that occurred during the Last Supper. Jesus explained, “The one who has bathed does not need to wash, except for his feet, but is completely clean. And you are clean...” John 13:10 (ESV) When we accept by faith the sacrifice of Jesus on the cross we are "completely clean." But the reality of everyday life causes us to occasionally need to wash our feet because we're walking every day through a world that is filthy with sin.
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