So then faith cometh by hearing, and hearing by the word of God. – Romans 10:17
We see by this conclusion what Paul had in view by the gradation which he formed; it was to show, that wherever faith is, God has there already given an evidence of his election; and then, that he, by pouring his blessing on the ministration of the gospel, to illuminate the minds of men by faith, and thereby to lead them to call on his name, had thus testified, that the Gentiles were admitted by him into a participation of the eternal inheritance.
And this is a remarkable passage with regard to the efficacy of preaching; for he testifies, that by it faith is produced. He had indeed before declared, that of itself it is of no avail; but that when it pleases the Lord to work, it becomes the instrument of his power. And indeed the voice of man can by no means penetrate into the soul; and mortal man would be too much exalted, were he said to have the power to regenerate us; the light also of faith is something sublimer than what can be conveyed by man: but all these things are no hindrances, that God should not work effectually through the voice of man, so as to create faith in us through his ministry.
It must be further noticed, that faith is grounded on nothing else but the truth of God; for Paul does not teach us that faith springs from any other kind of doctrine, but he expressly restricts it to the word of God; and this restriction would have been improper if faith could rest on the decrees of men. Away then with all the devices of men when we speak of the certainty of faith. Hence also the Papal conceit respecting implicit faith falls to the ground, because it tears away faith from the word; and more detestable still is that blasphemy, that the truth of the word remains suspended until the authority of the Church establishes it.
 John Calvin, Commentary on the Epistle of Paul the Apostle to the Romans, 401-02.