1 John 2:21

“I have not written unto you, &c.] Either this epistle, or rather what particularly here regards those apostates from the truth, in order to shun them and not be deceived by them: the apostle here obviates an objection that he saw might be made upon what he last said, that they knew all things; and, if so, why then did he write the things he did, since they knew them before? to which he answers, that he did not write to them as to ignorant, but as to knowing persons: because ye know not the truth, but because ye know it: the Father, who is the God of truth; Christ, who is truth itself; and the Spirit, who is the Spirit of truth; and the Gospel, which is the word of truth; and the Scriptures,…”

“but because ye know it, and that, &c.--Ye not only know what is the truth (concerning the Son and the Father, Jo1 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.”

“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 its 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…”

“I have written - Namely, Jo1 2:13. To you because ye know the truth - That is, to confirm you in the knowledge ye have already. Ye know that no lie is of the truth - That all the doctrines of these antichrists are irreconcilable to it. 1 John 2:22”

“because ye know not: Pro 1:5, Pro 9:8, Pro 9:9; Rom 15:14, Rom 15:15; Pe2 1:12 1 John 2:22”