Beloved, thou doest faithfully - In the previous verses the writer had commended Gaius for his attachment to truth, and his general correctness in his Christian life. He now speaks more particularly of his acts of generous hospitality, and says that he had fully, in that respect, done his duty as a Christian.
Whatsoever thou doest - In all your contact with them, and in all your conduct toward them. The particular thing which led to this remark was his hospitality; but the testimony respecting his general conduct had been such as to justify this commendation.
To the brethren - Probably to Christians who were well known to him - perhaps referring to Christians in his own church.
And to strangers - Such as had gone to the church of which he was a member with a letter of commendation from John. Compare the Romans 12:13 note, and Hebrews 13:2 note.

Thou doest faithfully - Πιστον ποιεις. Kypke thinks that πιστον is put here for πιστιν, and that the phrase signifies to keep or preserve the faith, or to be bound by the faith, or to keep one's engagements. Thou hast acted as the faith - the Christian religion, required thee to act, in all that thou hast done, both to the brethren at home, and to the strangers - the itinerant evangelists, who, in the course of their travels, have called at thy house. There is not a word here about the pilgrims and penitential journeys which the papists contrive to bring out of this text.

Beloved, thou doest (b) faithfully whatsoever thou doest to the brethren, and to strangers;
(b) As becomes a believer and a Christian.

Beloved, thou doest faithfully,.... Or a faithful thing, and as became a faithful man, a believer in Christ; in all his beneficence and charity he acted the upright part; he did not do it in an hypocritical way, to be seen of men, and gain applause from them, but from a principle of love, and with a view to the glory of God:
whatsoever thou doest to the brethren, and to strangers; which may design either different persons; and by "brethren" may be meant the poor brethren of the church that. Gaius belonged to, and others that were well known to him; and by "the strangers", not unconverted persons, but such of the saints as came from foreign parts, and travelled about to spread the Gospel, and enlarge the interest of Christ: or else the same persons may be intended, for the words may be read, as they are in the Alexandrian copy, and some others, and in the Vulgate Latin version, "what thou doest to the brethren, and this to strangers"; that is, as the Arabic version renders it, "to strange brethren"; or, as the Syriac version, "to the brethren, and especially them that are strangers"; so that Gaius was a very hospitable man, one that entertained and lodged strangers, and used them very civilly and courteously, with great liberality, and with much integrity and sincerity.

faithfully--an act becoming a faithful man.
whatsoever thou doest--a distinct Greek word from the former "doest": translate, "workest": whatsoever work, or labor of love, thou dost perform. So Matthew 26:10, "She hath wrought a good work upon me."
and to strangers--The oldest manuscripts, "and that (that is, and those brethren) strangers." The fact of the brethren whom thou didst entertain being "strangers," enhances the love manifested in the act.

Thou doest a faithful work. In his kindness to brethren and strangers. In those early ages hospitality to the saints sent on evangelistic missions, and especially to persecuted saints driven from home, was a very important duty.
Which have borne testimony to thy love. These traveling brethren reported to the Church how Gaius had aided them.
Thou shalt do well. A polite exhortation to continue such hospitality in the future. The hospitality was wont to be shown not only by opening the home to these saints, but by helping them on their journey.
Because. The persons named were engaged in the work of Christ (his name's sake).
Taking nothing. They could not ask aid of the Gentiles. The Greek form does not imply that the Gentiles had tendered presents which had been rejected (Winer's Greek Grammar of the New Testament, page 388, note 1).

Faithfully - Uprightly and sincerely.