Itinerant teachers sent out by John were rejected in one of the churches in the province of Asia by a dictatorial leader, Diotrephes, who even excommunicated members who showed hospitality to John’s messengers. John wrote this letter to commend Gaius for supporting the teachers and, indirectly, to warn Diotrephes.
This epistle is addressed to a converted Gentile. The scope is to commend his steadfastness in the faith, and his hospitality, especially to the ministers of Christ.
Matthew Henry