{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.

People's New Testament

2:4 He that saith, etc. If one claims to have a saving knowledge of Christ, yet lives in disobedience, his life puts the lie to his words.

Jamieson-Fausset-Brown Bible Commentary

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

Matthew Henry's Concise Commentary

2:3-11 What knowledge of Christ can that be, which sees not that he is most worthy of our entire obedience? And a disobedient life shows there is neither religion nor honesty in the professor. The love of God is perfected in him that keeps his commandments. God's grace in him attains its true mark, and produces its sovereign effect as far as may be in this world, and this is man's regeneration; though never absolutely perfect here. Yet this observing Christ's commands, has holiness and excellency which, if universal, would make the earth resemble heaven itself. The command to love one another had been in force from the beginning of the world; but it might be called a new command as given to Christians. It was new in them, as their situation was new in respect of its motives, rules, and obligations. And those who walk in hatred and enmity to believers, remain in a dark state. Christian love teaches us to value our brother's soul, and to dread every thing hurtful to his purity and peace. Where spiritual darkness dwells, in mind, the judgment, and the conscience will be darkened, and will mistake the way to heavenly life. These things demand serious self-examination; and earnest prayer, that God would show us what we are, and whither we are going.