Peter Leithart commenting on Diotrephes in III John 1:9-10 in his book From Behind the Veil.
Though the specific situation of Gaius’ church is not replicated in every church, the threat of prideful members and leaders like Diotrephes is a perpetual threat. John instructs churches of all times to pay attention and watch out for men in the church who lust for leadership. Watch out for the ambitious man. Watch out for men who refuse to bow to authority, take advice and counsel, resist necessary rebukes and exhortations. Watch out for the men and women in the church who fill the church with gossip and evil words about the leaders in the church. Watch out for people John says, who refuse to receive brothers, who close themselves off from Christians who are not part of the in group. Watch out for these Judaizing tendencies.
This is all the product of pride, of wishing to be first, and it destroys the fellowship of the church. This is evil conduct of the most serious nature. If we are going to be judged by how we treat the “least of these my brothers,” then we had better be very careful to avoid pride, ambition, gossip, slander, evil words, and resistance to counsel. We ought instead to be cultivating humility, service, edifying speech, and submission to the authority of those set over us.