We know nothing of Demetrius from 3 John. We can assume he is a missionary based on the context. Perhaps the leader of this particular group John calls the brothers. We can juxtapose his actions to those of Diotrephes, and believe he must be of God, imitating good (Christ) and living (walking) according to truth (the Gospel) in the same manner for which John praises Gaius. All we have is one verse which mentions him:

12: Demetrius has received a good testimony from everyone, and from the truth itself. We also add our testimony, and you know our testimony is true.

The power of testimony. 

According to this verse, everyone could prove he is indeed good. They have proof of his goodness by example of his actions. But good actions are not always Gods actions and so thankfully John even claims truth itself testifies for Demetrius. That the Holy Spirit (Truth) is the example of the goodness in this man. John has assumingely witnessed conversion, regeneration, progressive sanctification and now to the point of this man’s missional vocation. His faith overflows into his works. This was evidenced by one man (John) but also the other Elders (we). And their testimony is also approved by the Holy Spirit.

What a gift as a leader to see those whom we take on as our spiritual children flourish in the Spirit, and to send them off in testimony of the goodness of God.

What a gift as parents, to see our own children do the same. Thank you for your mercy, grace and kindness Lord, for this is not by my actions but yours. Amen.

Peace be with you.