A.T. Robertson Commentary 1 John 1:10

"If we say that we have not sinned, we make him a liar, and his word is not in us." — 1 John 1:10 (ASV)

If we say (εαν ειπωμεν). As in verses 6,8.

We have not sinned (ουχ αμαρτηκαμεν). Perfect active indicative of αμαρτανω. This is a denial of any specific acts of sin, while in verse 8 we have the denial of the principle of sin. David Smith observes that the claim to personal perfectionism has two causes, one the stifling of conscience in making God a liar (ψευστην, the word used of the devil by Jesus in Joh 8:44), and the other ignorance of God's word, which is not in us, else we should not make such a claim.