And when all the people saw it, they fell on their faces: and they said, The LORD, he is the God; the LORD, he is the God.

Wesley's Notes

18:39 They fell - In acknowledgment of the true God. He is God - He alone; and Baal is a senseless idol. And they double the words, to note their abundant satisfaction and assurance of the truth of their assertion.

Matthew Henry's Concise Commentary

18:21-40 Many of the people wavered in their judgment, and varied in their practice. Elijah called upon them to determine whether Jehovah or Baal was the self-existent, supreme God, the Creator, Governor, and Judge of the world, and to follow him alone. It is dangerous to halt between the service of God and the service of sin, the dominion of Christ and the dominion of our lusts. If Jesus be the only Saviour, let us cleave to him alone for every thing; if the Bible be the world of God, let us reverence and receive the whole of it, and submit our understanding to the Divine teaching it contains. Elijah proposed to bring the matter to a trial. Baal had all the outward advantages, but the event encourages all God's witnesses and advocates never to fear the face of man. The God that answers by fire, let him be God: the atonement was to be made by sacrifice, before the judgment could be removed in mercy. The God therefore that has power to pardon sin, and to signify it by consuming the sin-offering, must needs be the God that can relieve from the calamity. God never required his worshippers to honour him in the manner of the worshippers of Baal; but the service of the devil, though sometimes it pleases and pampers the body, yet, in other things, really is cruel to it, as in envy and drunkenness. God requires that we mortify our lusts and corruptions; but bodily penances and severities are no pleasure to him. Who has required these things at your hands? A few words uttered in assured faith, and with fervent affection for the glory of God, and love to the souls of men, or thirstings after the Lord's image and his favour, form the effectual, fervent prayer of the righteous man, which availeth much. Elijah sought not his own glory, but that of God, for the good of the people. The people are all agreed, convinced, and satisfied; Jehovah, he is the God. Some, we hope, had their hearts turned, but most of them were convinced only, not converted. Blessed are they that have not seen what these saw, yet have believed, and have been wrought upon by it, more than they that saw it.