Demetrius was highly regarded by everybody because of his virtue. Some people take this verse to mean that he was criticized by everyone, but I cannot see how such an interpretation is possible.
Demetrius has received a good testimony from everyone. That is, he himself welcomed the weak and supported the needy for the sake of the truth. Therefore, Gaius should also imitate him, so that he too may be worthy of the praise of all.
Demetrius has been testified by all, and by the very truth itself: but we also bear witness, and you know that our testimony is true.
"by all:" say those who keep the truth.
"and by the very truth itself:" which is the effectiveness of speech. For there are some to whom virtue bears witness, but it is frustrated by ineffective speech or lacking in deeds. If someone accepts the part “by all,” even from the unfaithful, because “all” comprehends everyone, he would not be wrong to take the speech in a good sense. It commands the Jews and Greeks and the Church of God to strive, just as I, John says, please all in all things." (1 Cor. 10:35)
[AD 449] Hilary of Arles on 3 John 1:12