This verse teaches us that we ought to bear the abuse of those who insult us with equanimity, but sometimes we have to protest it because if we do not do so, these people will corrupt the minds of those who might otherwise have heard something good about us.
I would have written to the church, but he who loves to have the preeminence among them, etc. Diotrephes, it seems, was an arch-heretic of that time, someone proud and insolent, preferring to usurp the preeminence of knowledge by teaching new doctrines, rather than humbly listening to the commands of the ancient holy Church, which John preached. Hence, aptly, Diotrephes is humorously derided, or interpreted as "mad elegance," to signify with his name even the perfidy of his heart.
I wrote to the church, but Diotrephes, who loves to be first among them, does not receive us. Therefore, if I come, I will bring to light the works he is doing, gossiping wickedly about us. And not content with that, he not only does not receive the brothers himself, but also forbids those who want to receive them and expels them from the congregation.
"I will bring to light the works he is doing." If it is commanded that evil should not be repaid with evil (Rom. 12:17), what is it that he now threatens? Therefore, we say that this command is so that we do not repay evil to him who has sinned only against ourselves: For if it is a sin against us, and it brings an obstacle to faith, it should be referred to such, as Paul did to Elymas who was perverting the ways of the Lord. (Acts 13:10)
"gossiping wickedly." That is, throwing insults and cursing.
Diotrephes, who loveth: This man seemeth to be in power, but not a friend to the faithful; therefore this part of the letter might be an admonition to him from the apostle.
[AD 449] Hilary of Arles on 3 John 1:9