‘Demetrius has the witness of all, and of the truth itself. Yes, we also bear witness: and you know that our witness is true.'
Demetrius may have been another church leader, or a more probably a visiting preacher, a representative of John, and he may well also have been the letter bearer. He too is a man who adorns the truth, witnessed to by all. He is to be welcomed. Even John bears witness to his godliness. And Gaius can know that his testimony is reliable. Thus he can know that he can place complete confidence in Demetrius. His coming would give Gaius comfort at a distressful time. John is very practical as well as spiritual.