1 John 2:21

I have not written unto you
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, which are the Scriptures of truth, and from whence truth is to be fetched, and by them to be confirmed and defended; and which, if they had not known, it would have been to no purpose for him to have written to them about the antichrists that were come into the world; and though they did know the truth, it was very proper to put them in remembrance of it, and to establish them in it, against these deceivers, which supposes former knowledge of it:

and that no lie is of the truth;
either springs from it, or is according to it, but just the reverse. The apostle has respect to the errors and heresies of the above apostates, which were flagrant contradictions to the Gospel, and as distant from it as a lie is to truth; and of such lies, and of those liars, he speaks in the next verses. The Arabic version reads, "and that every liar is not of the truth".