For I rejoiced greatly when the brethren came - Who these were is not certainly known. They may have been members of the same church with Gaius, who, for some reason, had visited the writer of this Epistle; or they may have been the "brethren" who had gone from him with a letter of commendation to the church, 3-John 1:9, and had been rejected by the church through the influence of Diotrephes, and who, after having been hospitably entertained by Gaius, had again returned to the writer of this Epistle. In that case, they would of course bear honorable testimony to the kindness which they had received from Gaius, and to his Christian character.
And testified of the truth that is in thee - That you adhere steadfastly to the truth, notwithstanding the fact that errors abound, and that there are many false teachers in the world.
Even as thou walkest in the truth - Livest in accordance with the truth. The writer had made the same remark of the children of Cyria, to whom the second Epistle was directed. See the notes at 2-John 1:4.

When the brethren came - Probably the same of whom he speaks in the fifth (3-John 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 2-John 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 (3-John 1:9).

For - I know thou usest all thy talents to his glory. The truth that is in thee - The true faith and love.