Adam Clarke on 1 John 3:6

Whosoever abideth in him - By faith, love, and obedience.

Sinneth not - Because his heart is purified by faith, and he is a worker together with God, and consequently does not receive the grace of God in vain. See on 1Jn 3:3 (note).

Hath not seen him - It is no unusual thing with this apostle, both in his gospel and in his epistles, to put occasionally the past for the present, and the present for the past tense. It is very likely that here he puts, after the manner of the Hebrew, the preterite for the present: He who sins against God doth not see him, neither doth he know him - the eye of his faith is darkened, so that he cannot see him as he formerly did; and he has no longer the experimental knowledge of God as his Father and portion.