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.

Robert Jamieson, A. R. Fausset & David Brown, 1871 · public domain

Verse 21. I have not written unto you because ye know not the truth. You are not to regard my writing to you in this earnest manner as any evidence that I do not suppose you to be acquainted with religion and it duties. Some, perhaps, might have been disposed to put this construction on what he had said, but he assures them that that was not the reason why he had thus addressed them. The very fact that they did understand the subject of religion, he says, was rather the reason why he wrote to them.

But because ye know it. This was the ground of his hope that his appeal would be effectual. If they had never known what religion was, if they were ignorant of its nature and its claims, he would have had much less hope of being able to guard them against error, and of securing their steady walk in the path of piety. We may always make a strong and confident appeal to those who really understand what the nature of religion is, and what are the evidences of its truth.

And that no lie is of the truth. No form of error, however plausible it may appear, however ingeniously it may be defended, and however much it may seem to be favourable to human virtue and happiness, can be founded in truth. What the apostle says here has somewhat the aspect of a truism, but it contains a real truth of vital importance, and one which should have great influence in determining our minds in regard to any proposed opinion or doctrine. Error often appears plausible. It seems to be adapted to relieve the mind of many difficulties which perplex and embarass it on the subject of religion. It seems to be adapted to promote religion. It seems to make those who embrace it happy, and for a time they apparently enjoy religion. But John says that however plausible all this may be, however much it may seem to prove that the doctrines thus embraced are of God, it is a great and vital maxim that no error can have its foundation in truth, and, of course, that it must be worthless. The grand question is, what is truth; and when that is determined, we can easily settle the inquiries which come up about the various doctrines that are abroad in the world. Mere plausible appearances, or temporary good results that may grow out of a doctrine, do not prove that it is based on truth; for whatever those results may be, it is impossible that any error, however plausible, should have its origin in the truth.

Albert Barnes, 1834 · public domain

Verse 21 I have not written, etc. - It is not because ye are ignorant of these things that I write to you, but because you know them, and can by these judge of the doctrines of those false teachers, and clearly perceive that they are liars; for they contradict the truth which ye have already received, and consequently their doctrine is a lie, and no lie can be of the truth, i.e. consistent with Christianity.

Adam Clarke, 1831 · public domain