...Profane men think that religion rests only on opinion, and, therefore, that they may not believe foolishly, or on slight grounds, desire and insist to have it proved by reason that Moses and the prophets were divinely inspired. But I answer,that the testimony of the Spirit is superior to reason. For as God alone can properly bear witness to his own words, so these words will not obtain full credit in the hearts of men, until they are sealed by the inward testimony of the Spirit. The same Spirit, therefore, who spoke by the mouth of the prophets, must penetrate our hearts, in order to convince us that they faithfully delivered the message with which they were divinely entrusted. (Calvins Institutes, Chapter 7, Section 4)
It's a fallacy to believe that the authority of God's Word rests upon the opinion of man. Readers should note however, Calvin goes on in chapter 8 to provide rational proofs (for the credibility of Scripture) for use when they may be beneficial.
(Thanks to the Center for Reformed Theology and Apologetics for the reference to the Institutes)