He that saith, I know him How does he prove that they are liars who boast that they have faith without piety? even by the contrary effect; for he has already said, that the knowledge of God is efficacious. For God is not known by a naked imagination, since he reveals himself inwardly to our hearts by the Spirit. Besides, as many hypocrites vainly boast that they have faith, the Apostle charges all such with falsehood; for what he says would be superfluous, were there no false and vain profession of Christianity made by man.

He that saith, I know Him - He who professes to be acquainted with the Saviour, or who professes to be a Christian.
And keepeth not his commandments - What he has appointed to be observed by his people; that is, he who does not obey him.
Is a liar - Makes a false profession; professes to have that which he really has not. Such a profession is a falsehood, because there can be no true religion where one does not obey the law of God.

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.

(3) He that saith, I know him, and keepeth not his commandments, is a liar, and the truth is not in him.
(3) Holiness, that is, a life ordered according to the prescript of God's commandments, however weak we are, is of necessity joined with faith, that is, with the true knowledge of the Father in the Son.

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 1-John 1:8.

I know--Greek, "I have knowledge of (perfect) Him." Compare with this verse 1-John 1:8.