46 The interpretation of the three ὅτι clauses in v. 21 is very difficult: (1) All three instances of ὅτι (hoti) may be causal (so , , ). (2) The first two may be causal while the third indicates content (declarative or recitative ὅτι, so , , , ). (3) However, it is best to take all three instances as indicating content because this allows all three to be subordinate to the verb ἔγραψα (egrapsa) as compound direct objects. The author writes to reassure his readers (a) that they do indeed know the truth (first two uses of ὅτι) and (b) that no lie is of the truth (third use).
47 See the note on the first occurrence of “that” in v. .
48 See the note on the first occurrence of “that” in v. .

In other versions

I write to you, not because you do not know the truth, but because you know it, and because no lie is of the truth.

I do not write to you because you do not know the truth, but because you do know it and because no lie comes from the truth.

I have not written to you because you don’t know the truth, but because you do know it, and because no lie comes from the truth.

So I am writing to you not because you don’t know the truth but because you know the difference between truth and lies.

I have not written to you because you do not know the truth, but because you do know it, and because every lie is not of the truth.

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.

Public Domain