Jamieson, Fausset and Brown on 1 John 2:21

21. but because ye know it, and that, &c.--Ye not only know what is the truth (concerning the Son and the Father, 1Jo 2:13), but also are able to detect a lie as a thing opposed to the truth. For right (a straight line) is the index of itself and of what is crooked [Estius]. The Greek is susceptible of Alford's translation, "Because ye know it, and because no lie is of the truth" (literally, "every lie is excluded from being of the truth"). I therefore wrote (in this Epistle) to point out what the lie is, and who the liars are.