John Calvin 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)

And that no lie is of the truth. He concedes to them a judgment by which they could distinguish truth from falsehood. For this is not the dialectical proposition that falsehood differs from truth (such as are taught as general rules in the schools), but what is said is applied to what is practical and useful. It is as if he had said that they not only held what was true, but were also so fortified against the impostures and fallacies of the ungodly that they wisely guarded themselves.

Furthermore, he is not speaking of this or that kind of falsehood. Instead, he says that whatever deception Satan might contrive, or in whatever way Satan might attack them, they would be readily able to distinguish between light and darkness, because they had the Spirit as their guide.