I have written something to the church, but Diotrephes, who likes to put himself first, does not acknowledge our authority. So if I come, I will bring up what he is doing, talking wicked nonsense against us. And not content with that, he refuses to welcome the brothers, and also stops those who want to and puts them out of the church. —3 John 1:9-10 (ESV)
Diotrephes had ambition and drive, and he had made it all the way up the ladder and was now the ruler of the church. He loved being at the top of the organizational chart (the word used to describe him is φιλοπρωτεύω, which basically means “lover of being first”), where he was in command and could control others. He had big plans, and they did not include sharing the stage with or supporting visiting believers. He did his own thing, refusing to walk in humility under the authority of the apostles, and doing whatever he needed to maintain his power.
John makes it clear that this is not doing good, it is evil. Not just a sinful slip-up, but evidence that Diotrephes actually has no clue about who God even is:
Beloved, do not imitate evil but imitate good. Whoever does good is from God; whoever does evil has not seen God. —3 John 1:11 (ESV)
Loving power, ruling over others, and playing the political game to stay in control is dishonoring to God. Which makes the contrast of Diotrephes with Demetrius so stark:
Demetrius has received a good testimony from everyone, and from the truth itself. We also add our testimony, and you know that our testimony is true. —3 John 1:12 (ESV)
If Diotrephes represents the top of a traditional power structure, Demetrius represents a hub in an open web of trust. He does not have organizational power of position, but what he does have is far more important: the trust of others. He has a good testimony—a “stamp of approval”—from everyone in the church, making him a leader and influencer.
Not only does Demetrius receive a good testimony from everyone who knows him, but he receives a good testimony from the truth itself. Meaning: what Demetrius teaches and does is done with integrity, in the open, where all can see that it is in line with the Truth of God’s Word. Demetrius is not playing a power game of withholding information to leverage power for his own advance. He lives openly, humbly, and honestly. So in addition to the testimony of the people who know him and the testimony of the truth itself, John adds the testimony of the apostles to the affirmation of Demetrius’ good character.