A.T. Robertson Commentary 1 John 1:8

"If we say that we have no sin, we deceive ourselves, and the truth is not in us." — 1 John 1:8 (ASV)

If we say (εαν ειπωμεν). See verse 6.

We have no sin (αμαρτιαν ουκ εχομεν). For this phrase see Joh 9:41; 15:22,24. That is, we have no personal guilt, no principle of sin. This some of the Gnostics held, since matter was evil and the soul was not contaminated by the sinful flesh, a thin delusion with which so-called Christian scientists delude themselves today.

We deceive ourselves (εαυτους πλανωμεν). Present active indicative of πλαναω, to lead astray. We do not deceive others who know us. Negative statement again of the same idea, "the truth is not in us."