John Lightfoot on John 7:28

[He that sent me is true, whom ye know not.] "The men of Judea may be credited as to the purity of the wine and the oil." Gloss: "Even the people of the land, the very vulgar sort, may be credited for the purity of the wine and the oil, which is dedicated by them to the altar in the time of the vintage or pressing."

Men not known by name or face to the priests, yet if they offered wine or oil, were credited as to the purity and fitness of either, from their place of habitation. There are numberless instances of men, though perfectly unknown, yet that may be credited, either as to tithes, or separating the Trumah, or giving their testimony, &c. To the same sense our Saviour, chapter 5:31, "If I bear witness of myself, my witness is not true"; i.e. in your judicatories it is not of any value with you, where no one is allowed to be a witness for himself. And in this place, "'He that hath sent me,' although you know him not, yet 'is he true, or worthy belief,' however I myself may not be so amongst you."