John Calvin Commentary 1 John 3:6

"Whosoever abideth in him sinneth not: whosoever sinneth hath not seen him, neither knoweth him." — 1 John 3:6 (ASV)

Whoever sins has not seen him. According to his usual manner, he added the opposite clause, so that we may know that faith in Christ and knowledge of him are claimed in vain unless there is newness of life. For Christ is never dormant where he reigns, but the Spirit makes his power effectual. And it may be rightly said of him that he puts sin to flight, just as the sun drives away darkness by its own brightness.

But we are again taught in this place how strong and efficacious the knowledge of Christ is, for it transforms us into his image. So by seeing and knowing we are to understand nothing other than faith.