He who saith that he knoweth him, and keepeth not his commandments, is a liar, and the truth is not in him.

“It is the manner of life which shows up those who know the commandments; for as a man's word is, so is his life. The tree is known by its fruit, not by its blossom and leaves. Knowledge, then, comes from the fruit and from behaviour, not from talk and from blossom. We say that knowledge is not mere talk, but a certain divine knowledge, that light which is kindled in the soul as a result of obedience to the commandments, and which reveals all that is in a state of becoming, enables man to know himself and teaches him to become possessed of God. What the eye is in the body, knowledge is in the mind. Let them not call bondage to pleasure freedom, as if bitterness were sweet. We have learnt to recognize as freedom that which the Lord alone confers on us when he liberates us from lusts and desires and the other passions. "He who says, I know the Lord, and does not keep his commandments, is a liar and the truth is not in him," says John.”

“"And in this we do know Him, if we keep His commandments." What commandments? "He that saith, I know Him, and keepeth not His commandments, is a liar, and the truth is not in him." But still thou askest, What commandments? "But whoso," saith he, "keepeth His word, in him verily is the love of God perfected." Let us see whether this same commandment be not called love. For we were asking, what commandments, and he saith, "But whoso keepeth His word, in him verily is the love of God perfected." Mark the Gospel, whether this be not the commandment: "A new commandment," saith the Lord, "give I unto you, that ye love one another."”

“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.”

“He that saith, I know him - This is a severe blow against those false teachers, and against all pretenders to religious knowledge, who live under the power of their sins; and against all Antinomians, and false boasters in the righteousness of Christ as a covering for their personal unholiness. They are all liars, and no truth of God is in them.”