Exegetical Insight:

“οἴδατε” is perfect indicative active[1]. This is present tense knowledge. It is not that someone knows something in the past and that is good enough. John is able to write these things to his readers because they currently possess a knowledge that allows them to discern truth from error (lies).

As we know, this indicates a current state of relational knowing of Jesus Christ. As verses eighteen and nineteen indicate, there were some who were, for a time, a part of the group. However, they then followed after different “ἀντίχριστος”, or antichrists. The truth was not in them; they did not “οἴδατε” , or know the truth. This truth is found in verse thirteen and verse twenty-two. The truth is they “νενικήκατε τὸν πονηρόν” (verse thirteen). The lie the “ἀντίχριστος” peddles is in verse twenty-two, “ἀρνούμενος τὸν πατέρα καὶ τὸν υἱόν”, anyone who denies God the Father and Jesus His Son.

 

So this tells us that John is able to tell them truths because they know who the Father and the Son of God are; this enables them to know the truth.