If we say we have not sinned, we make Him out to be a liar, and His word is not in us.
10 We make him a liar He goes still further, that they who claim purity for themselves blaspheme God. For we see that he everywhere represents the whole race of man as guilty of sin. Whosoever then tries to escape this charge carries on war with God, and accuses him of falsehood, as though he condemned the undeserving.
To deny our sin is to deceive ourselves and exclude the truth from us entirely. The Christian religion is the religion of sinners; Christian life is continued repentance and mortification of sin. Denying sin does not merely deceive us, it calls God a liar and dishonors His testimony against the world's sin.
AI summary
Commenting on 1 John 1:8-10
If we say that we have not sinned,.... Have never sinned, in time past as well as now; deny original sin, and that men are born in sin, but affirm they come into the world pure and holy; and assert that concupiscence is not sin; and so not regarding internal lusts and desires as sinful, only what is external, fancy they have so lived as...