The Spirit superior to reason

John Calvin, though clearly a great mind himself, recognizes that ultimately we cannot rely on rational proofs and only human reason to convinces us of the veracity of God's Word. We certainly need to use our minds and reason, but we are unable to comprehend fully without God's Spirit:

... 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. (Institutes 1.5.4)