Expositor's Bible Commentary Commentary 1 John 2:21

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

Lest there be any doubt among the faithful as to John’s perception of their understanding and orthodoxy, he says that he has not written because they did not know the truth—he himself is not providing any new information or teaching —but because they know it so certainly. They know the character of truth, and therefore they know that “no lie comes from truth.” John aims his remarks precisely at the innate knowledge of the Gospel that he knows his followers possess. Lies cannot come from God. The antichrists and their followers, the false teachers, are liars. So they do not come—in fact, they cannot come—from God, who is the truth.