3 John 1:10

The apostle promised and warned that whenever he might visit that congregation he would point out Diotrephes’ sinful behavior, assuming it continued. Specifically, Diotrephes was charging John falsely to elevate himself. Worse than that he was not giving hospitality to visiting brethren, as Gaius was, perhaps because he saw them as a threat to himself. Third, he intimidated others in the church and forced them to stop welcoming these men.

"Diotrephes was condemned not because he violated sound teaching regarding the person and nature of Jesus Christ but because his ’life’ was a contradiction to the truth of the gospel." [Note: Glenn W. Barker, "3 John," in Hebrews-Revelation, vol. 12 of The Expositor’s Bible Commentary, p. 375.]

"The verb ekballei, in the present tense again (literally, ’he throws out’), need not imply formal excommunication from the Church, as this became known later. Cf. Matthew 18:17; Luke 6:22; John 9:34-35; 1 Corinthians 5:2. On the other hand, it seems as if Diotrephes had already arrogated to himself the task of ’expulsion,’ and was actually driving people out of the congregation (as he had refused to welcome the brothers) rather than merely desiring to do so . . ." [Note: Smalley, p. 358.]

Obviously Gaius did not bow to his wishes, showing that he had strength of character and probably influence in the church. With this epistle John threw his support behind Gaius and against Diotrephes.