John Gill

He that saith I know him,.... God or Christ, as the Gnostics did, who pretended to great, even perfect, knowledge of divine things: and keepeth not his commandments; which the above persons had no regard to, and as many who profess great light and knowledge in our days show no concern for: is a liar; he contradicts what he says, and gives the lie to it; for though in words he professes to know God, in works he denies him, and which betrays his ignorance of him: and the truth is not in him; there is no true knowledge of God and Christ in him; nor is the truth of the Gospel in his heart, however it may be in his head; nor is the truth of grace in him, for each of these lead persons to obedience. The Ethiopic version renders it, "the truth of God is not with him"; See Gill on Jo1 1:8.