What Is the Reformed Doctrine of Human Reason?

What is the relationship between reason and faith? Is reason useful to mankind in learning of his God? This is sort of a hot topic in some Christian circles right now. We know that no one can be saved by logical deduction, but only by faith in the Lord Jesus Christ. Why? Because man’s reason could never discover the great mystery of salvation. Does that mean that reason doesn’t matter, or is so absolutely corrupted by sin so as to be useless? What about believers? How could such a gift be useless to those who have been born again, remade into the image of their Creator? What is the historic, Reformed approach to these questions? I was recently led to an old, dead guy who has elegant and learned things to say about all this:

Reason and Faith may kisse each other. There is a twin-light springing from both, and they both spring from the same foundation of light, and they both sweetly conspire in the same end, the glory of that being from which they shine, & the welfare & the happines of that being upon which they shine. So that to blaspheme Reason, ‘tis to reproach heaven it self, and to dishonour the God of Reason, to question the beauty of his Image, and by a strange ingratitude to slight this great and Royal gift of our Creator. For ‘tis he that set up these two great Luminaries in every heavenly soul, the Sun to rule the day, and the Moon to rule the night; and though there be some kinde of creatures that will bark at this lesser light, and others so severely critical, as that they make mountains of those spots and freckles which they see in her face, yet others know how to be thankful for her weaker beams, and will follow the least light of Gods setting up, though it be but the Candle of the Lord [Prov 20:27].

Nathaniel Culverwell, An Elegant and Learned Discourse of the Light of Nature (Liberty Fund, 2001), 10-11.