John Gill

Ye do the deeds of your father,.... Not Abraham, but the devil. Then said they unto him, we be not born of fornication: meaning either literally, that they were not a brood of bastards, children of whoredom, illegitimately begotten in unlawful copulation, or wedlock; or figuratively, that they were not the children of idolaters, idolatry being called fornication in Scripture; but that they were the holy seed of Israel, and children of the prophets, who had retained the pure word, and the true worship of God, though in all this they might have been contradicted and refuted; to which they add, we have one Father, even God; Israel being called by God his Son, and firstborn to them belonged the adoption, in a national sense, and of this they boasted; though few of them were the children of God by special adoption, or God their Father by regenerating grace.