We have only this verse where Demetrius is mentioned in the Bible, so this is all we know about him. He may have been the bearer of this letter, or one of the itinerant preachers who Diotrephes was not receiving. His name, meaning "belonging to Demeter," that is, Ceres, the goddess of agriculture, identifies him as a convert from paganism.
He had a good report from all men; the truth bore witness to him, and even John himself. He was true to the doctrine of Scripture, one of the shining lights of the New Testament, a humble saint of God.
Around us today, there are multitudes of people like him. They are not a Diotrephes, not even a Gaius, but just humble saints of God, doing the thing God has called them to do. Although they don't try to be the main speaker, to preside, to be the chairman of every board in the church, they are pillars of the church, supporting the work, and encouraging the preacher. Millions have lived for God unknown to the world, and have died unknown to the world. But they are known to God, and their names are inscribed in the Lamb's Book of Life.
Gaius, Diotrephes, and Demetrius: these are the three men who pass before us in this little letter. Two of these men were genuine and exemplary children of God. But one was false and domineering, yet a leader (self-appointed?) in the local church. May we beware of false leaders ourselves.
Though he wrote the Gospel of John and the Book of Revelation, two of the longest books of the New Testament, in closing this letter John very frankly said he would much rather speak face to face than write with pen and ink. We look forward to that, one day, in heaven!
12 Deme'trius has testimony from every one, and from the truth itself; I testify to him too, and you know my testimony is true.
13 I had much to write to you, but I would rather not write with pen and ink;
14 I hope to see you soon, and we will talk together face to face.
15 Peace be to you. The friends greet you. Greet the friends, every one of them.