What does 1 John 2:21 mean?

John again uses the phrase, "I write to you" (1 John 2:13–14), likely to emphasize the following words. His emphasis seems to be that those reading his letter are not antichrists. Truth is something a believer knows, not simply factual information, but knowledge that is accepted and applied. This, in fact, is faith: more than mere agreement, it is trust.

This verse also emphasizes the biblical view of truth. Namely, that truth exists and its opposite is false. Some approaches to truth deny this, claiming that opposites can both be true, or that there is no "real" truth. According to the Scriptures, this is not the case. There is one, and only one, "real" truth.

John follows with the clarification that truth and lies are in opposition to one another. This verse repeats the theme of 1 John 1:6 that living sinfully while claiming fellowship with God is a heinous lie. Truth is major theme in 1 John (1 John 1:6, 8; 2:4, 21; 3:18, 19; 4:6; 5:6), as well as 2 John (2 John 1:1, 2, 3, 4) and 3 John (3 John 1:1, 3, 4, 8, 12). John further mentions "truth" more than 20 times in his Gospel, emphasizing Jesus as truth (John 14:6) and that the truth will set you free (John 8:32).