This brief letter of John was apparently written and sent in haste to counter the actions and teaching of Diotrephes. John had much more to say, but wrote that he planned to visit the recipient of his letter, Gaius, and would address in person the other topics he had in mind. Nevertheless, the actions of Diotrephes apparently required an urgent and immediate response.
Diotrephes was a self-serving leader who appears to have been creating a division in the church. He was not a servant leader, but was rather seeking to elevate himself above the other leaders. Further, he was forbidding the members of the church to be hospitable to traveling ministers who were ministering in the name of Jesus.
John confronts Diotrephes, while being careful not to make any of the same missteps. John does not appeal to his superior authority, but rather invites the local church to make their own decision based on the facts, the testimony of reliable people, and the truth of God.