This is how we know that we know him: if we keep his commandments.
if we keep his commandments: John puts their knowledge to the test with a big conditional if. One may claim a knowing and intimate relationship with the Lord, but can he meet the test? One positive test that John sets forth is obedience to the commandments of God. One who does not keep the commandments does not truly know the Lord. "Keep" is tereo and means "to attend to carefully, to guard, observe" (Wuest, I John 112). The word carries a very serious connotation, impressing us with the solemnity of obeying the Lord. Serving God is serious business. The Christian who really knows the Lord shrinks in reverential fear at the prospect of disobeying Him. "Keepeth" is also in the present tense in the Greek and speaks of continuous action in observing God’s rules for living. Wuest translates this phrase, "if we are continually having a solicitous, watchful care in keeping His precepts" (113).