1 John 1:7 But if we walk in the light, as he is in the light, we have fellowship one with another, and the blood of Jesus Christ his Son cleanseth us from all sin.
8 If we say that we have no sin, we deceive ourselves, and the truth is not in us.
9 If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness.
10 If we say that we have not sinned, we make him a liar, and his word is not in us.
If the question of sin does not arise for a believer (since all his sins have been expiated on the Cross), then why are we told to confess our sins in verse 9?
The answer is that we are not told to confess our sins.
We are told that "if we confess our sins, He is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness."
This refers, not to justification, but to relationship. The whole letter is about relationship.
When we sin, our consciousness that we have sinned causes a strained, distant, cold relationship between us and God.
That is, from our side of the relationship. (From God's side, while we were yet sinners Christ died for us.)
From our side, we no longer enjoy praying, worshiping, reading God's Word...
If we confess our sins, God is faithful and just. He has already paid the penalty for our sins through His Son on the Cross.
But He will wipe that sin from our consciousness of strained relationship.
We can resume our relationship with God, just as if we had never sinned.