Book of 1 John Chapter 2 Vs. 21

 Warning Concerning Antichrists

1 John 2:21 "I have not written unto you because ye know not the truth, but because ye know it, and that no lie is of the truth."

I have not... I write to confirm the truth you already know, and to warn you against those who would seduce you (1Jhn. 2:26-27).

I have not written (οὐκ ἔγραψα)

Or I wrote not. Or, strictly, I wrote. Compare I write (1Jhn. 2:12, 2:13), and note the change of tense. The past tense, I wrote, does not refer to some previous writing, as the Gospel, but, like the present, to this Epistle. The present, I write, refers to the immediate act of writing: the aorist is the epistolary aorist, by which the writer places himself at the reader's stand-point, regarding the writing as past. See on 1Pet. 5:12. I write, therefore, refers to the Apostle's immediate act of writing; I have written, or I wrote, to the reader's act of reading the completed writing.

As a result of their anointing, the readers perhaps primarily the church leaders had adequate instruction in the truth of God. John wrote them precisely because their apprehension of the truth was correct and because… the truth should never be confused with a lie.