We therefore ought to receive such - All of us ought hospitably to entertain and help such persons. The work in which they are engaged is one of pure benevolence. They have no selfish aims and ends in it. They do not even look for the supplies of their own needs among the people to whom they go to minister; and we ought, therefore, to help them in their work, and to contribute to their support. Doubtless, the apostle meant to urge this duty particularly upon Gaius; but, in order to show that he recognized the obligation himself, he uses the term "we," and speaks of it as a duty binding on all Christians.
That we might be fellow-helpers to the truth - All Christians cannot go forth to preach the gospel, but all may contribute something to the support of those who do; and in this case they would have a joint participation in the work of spreading the truth. The same reasoning which was applicable to that case, is also applicable now in regard to the duty of supporting those who go forth to preach the gospel to the destitute.

We therefore ought to receive such - Those who are persecuted for righteousness' sake, and have professed the truth at the hazard of their lives, and the loss of all their worldly substance. Instead of απολαμβανειν, to receive, the most ancient and reputable MSS. have ὑπολαμβανειν, to take up, undertake for, or kindly receive.
Fellow helpers to the truth - And thus encourage the persecuted, and contribute to the spread and maintenance of the Gospel.

We therefore ought to receive such, that we might be (d) fellowhelpers to the truth.
(d) That we ourselves may help the preaching of the truth.

We therefore ought to receive such,.... We who are Jews, that have believed in Christ, for such an one Gaius, it seems, as well, as the apostle, were, ought to receive such preachers of the word into our houses, and entertain them cheerfully, while they continue, and supply them with all necessaries when they depart:
that we might be fellow helpers to the truth; that is, to the Gospel, and the propagation of it in the world: some are helpers to it, in preaching of it, by making use of the ministerial gifts bestowed upon them; and others are fellow helpers with them, to the same good work, by their purses, communicating freely to the support of those, who labour in the word and doctrine; and these latter have the honour to be co-workers, or fellow labourers with the former, as the word here used signifies. The Alexandrian copy reads, "fellow helpers to the church", that so the whole burden of taking care of these ministers might not lie upon them. The Vulgate Latin, Arabic, and Ethiopic versions read, "fellow helpers of the truth".

We--in contradistinction to "the Gentiles" or "heathen" referred to, 3-John 1:7.
therefore--as they take nothing from the Gentiles or heathen.
receive--The oldest manuscripts read, "take up." As they "take" nothing from the Gentiles, we ought to take them up so as to support them.
fellow helpers--with them.
to the truth--that is, to promote the truth.

To receive - With all kindness. The truth - Which they preach.