If we say we have no sin, we deceive ourselves, and the truth is not in us.
The confession of sins is a necessary consequence of "walking in the light" (Jo1 1:7). "If thou shalt confess thyself a sinner, the truth is in thee; for the truth is itself light. Not yet has thy life become perfectly light, as sins are still in thee, but yet thou hast already begun to be illuminated, because there is in thee confession of sins" [AUGUSTINE].
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 no sin,.... Notwithstanding believers are cleansed from their sins by the blood of Christ, yet they are not without sin; no man is without sin: this is not only true of all men, as they come into the world, being conceived in sin, and shapen in iniquity, and of all that are in a state of unregeneracy, and of...