III John: 9-13 Seeking What Is Good.
John indicates that he had previously written to the church that Gaius had some association with. It could be that he was referring to one or both of his other letters, since it was common practice to have them circulated, especially those that found their way into the canon of Scripture (v. 9a). Diotrephes may have been one of the false teachers that John had opposed. In any case, John makes quite clear that this man was self-seeking. Not only did he refuse to receive John and his colleagues and friends, but he openly prated against them, which means he prated against apostolic truth (vv. 9b-10 Cf. Prov. 10:8-10).
Again, John addresses his audience as ‘beloved’, and he warns them not to imitate this evil, but to imitate what is good. Just as he who loves is of God, even so they who do good are also of God, who is good (v. 11 Cf. Pss. 34:14; 37:27; I Th. 5:15; I Jn. 2:29; 3:10). That which is good is that which is true. On the other hand, Demetrius was one who had a good testimony, and this was an important thing to have, especially from “the truth itself” (v. 12a). With this testimony John bore witness that this testimony was true (v. 12b). These letters were important and necessary, but John ultimately wanted to meet face to face, and so he concludes by sending peace and greetings from their shared friends to the friends with Gaius, by name.