Demetrius was the opposite of Diotrephes. Demetrius was a clear example of a person who knows the truth and lives according to it (1:3). His name appears at the end of the letter. This suggests that he was the person who carried John’s letter to Gaius (compare Colossians 4:7–9; 1 Peter 5:12).
you know that our testimony is true: John wanted to protect his honor as a trustworthy elder for these churches. He did not want Diotrephes or other false teachers to challenge or replace his leadership.