Theophylact of Ohrid

"Not as a new commandment." And this is in agreement with what is said in the first epistle (1 John 2:7). He also adds the requirement of the commandment: "that we love one another." And he explains concerning love that its essence consists in walking according to it, and the commandment concerning it is a commandment from the beginning, and was given for nothing other than that you should walk according to it.

Commentary on 2 John ↗

Oecumenius

John said: "not as writing a new commandment to you," and this he firmly states to those who are mentioned in the preceding epistle, simultaneously demonstrating that it shows what they themselves had experienced by acting according to it, and not from difficult and foreign matters, affecting them with sorrow by approaching the unfamiliar. "not as writing a new commandment to you." He also adds the work of the commandment, that we love one another, and he explains that love consists in that we love one another.

Commentary on 2 John ↗

Bede

And now I ask you, lady, not as though I were writing a new commandment to you, etc. By this word, he reproaches the heretics, who were attempting to introduce new doctrines, leaving behind what they had heard from the apostles, and thereby disrupting the bonds of brotherly charity. Therefore, he says that he is by no means writing a new commandment but merely urging that the old faith and unblemished charity remain in all things.

Commentary on the Catholic Epistles ↗