William Burkitt on 3 John 1:9

Observe here, 1. The pious care which St. John took for the relief and succour of such faithful Christians as now travelled amongst them, both to avoid persecutation, and to preach the gospel; he wrote to the church on their behalf, desiring their reception, and advising their relief; I wrote to the church, that is, I wrote for them, and sent my testimonial to the church on their behalf; every one has a pen to plead for himself, happy he that has both tongue and pen to intercede for others.

Observe, 2. The opposition which St. John met with in so good a work; Diotrephes, a proud man, regarded not his letters, acknowledged not his authority, yea, slighted the apostle, prating against him with malicious words: the holiest men may meet with opposition in the holiest and best of actions, wherein the glory of God and the public good are most concerned. I wrote to the church, but Diotrephes received us not.

Observe, 3. The holy apostle's resentment of this indignity, and wise resolution thereupon: When I come, I will remember his deeds: that is, I will sharply rebuke him, and use that severity towards him which his crime deserves, according to the authority which God has given me.

Learn hence, That though private offences against Christ's ministers must be forgiven and forgotten by them, yet when an offence is prejudicial to the church, it must be opposed, and openly censured.