This is the conclusion of the former gradation, Rom_10:14. He speaketh here of the ordinary means whereby faith is wrought; not confining or limiting the Spirit of God, who worketh, or may work, by extraordinary means, yea, without any means at all. See Poole on "Rom_10:14".
By the word of God; by the command of God: q.d. The gospel could not be lawfully preached to them, for them to hear it, but by God’s command; and therefore the apostles and others, in preaching the gospel to the Gentiles, had good authority for what they did.