I. John receives a good report of Gaius's faith and charity from the brethren. Gaius’s love for the brethren and hospitality to strangers proves his sincerity in the faith. II. Gaius is commended for his hospitality, a virtue extended to strangers and those of different churches. III. John expresses great joy in hearing of Gaius's walk in truth. This reflects the joy of ministers when they see their followers living out their faith. IV. John advises Gaius to continue supporting the brethren by aiding them on their journeys in a godly manner, reflecting Christian generosity. V. The brethren went forth preaching for God's sake, taking nothing from the Gentiles. John emphasizes the need for the church to support such ministers, as they help propagate the truth.

When the brethren came - Probably the same of whom he speaks in the fifth (Jo3 1:5) and following verses, and who appear to have been itinerant evangelists.
The truth that is in thee - The soundness of thy faith and the depth of thy religion.

For I rejoiced greatly when the brethren came,.... From the place where Gaius lived, to that where John now resided; these brethren seem to be preachers of the word, who travelled from place to place to spread the Gospel:

and testified of the truth that is in thee; either of Christ, who was formed in him; or of the Gospel, which had a place in his heart; or of the truth of grace that was in him, as well as of that faithfulness, integrity, and sincerity he appeared to be possessed of, being an Israelite indeed, and without guile:

even as thou walkest in the truth: in Christ, and in the Gospel, and as became it, and with all uprightness; see Jo2 1:4; and this occasioned great joy in the apostle; as it is matter of joy to every one that truly loves Christ, and his Gospel; or has the true grace of charity in him, which envies not the gifts and graces of others, but rejoiceth in the truth, wherever it is found; and especially to the faithful ministers of the word, when they hear of the truth of grace in any souls, and that such continue walking in the truth of the Gospel, and particularly those who have been wrought upon under their ministry, as follows.

testified of the truth that is in thee--Greek, "of" (or 'to') thy truth": thy share of that truth in which thou walkest [ALFORD].

even as thou--in contrast to Diotrephes (Jo3 1:9).