Adam Clarke on 3 John 1:5

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.