If we say that we have not sinned, we make him a liar, and his word is not in us.
Parallel to 1 John 1:8.
We have not sinned - commission of actual sins, even after regeneration and conversion; whereas in 1 John 1:8, "we have no sin," refers to present GUILT, remaining (until cleansed) from actual sins committed, and to the SIN of our old nature still adhering to us. The perfect "have ... sinned" brings down the commission of sins to the present time: not merely sins before, but since, conversion.
We make him a liar - a gradation: 1 John 1:6, "we lie;" 1 John 1:8, "we deceive ourselves;" worst of all, "we make Him a liar," by denying His Word that all are sinners (cf. 1 John 5:10).
His word is not in us. "His word," "the truth" (1 John 1:8), accuses us truly; by denying it we drive it from our hearts (cf. John 5:38). Our rejection of it, as to our being sinners, involves our rejection of His word and will revealed in the Law and Gospel as a whole; for these throughout rest on the fact that we have sinned, and have sin.