Expositor's Bible Commentary Commentary 1 John 2:4

"He that saith, I know him, and keepeth not his commandments, is a liar, and the truth is not in him;" — 1 John 2:4 (ASV)

Now the author addresses his opponents who claim that they know God but at the same time break his commandments. Since he considers knowledge of God practical and experiential, to claim to know God and at the same time to disobey his commandments is to lie and be devoid of all truth.