Everyone speaks well of Demetrius, and the truth itself speaks for him. We vouch for him too, and you know that what we say about him is true.
In context
10So if I come, I will bring up what he is doing — spreading malicious gossip about us. And as if that were not enough, he refuses to welcome the brothers and sisters himself, stops anyone who wants to welcome them, and throws them out of the church. 11Dear friend, do not imitate what is evil, but what is good. Whoever does good belongs to God; whoever does evil has never seen God. 12Everyone speaks well of Demetrius, and the truth itself speaks for him. We vouch for him too, and you know that what we say about him is true. 13I have much more I could say to you, but I would rather not trust it to pen and ink. 14But I hope to see you soon, and then we can talk face to face. Peace be with you. The friends here send their greetings. Greet each of our friends there by name.