You find this notion originating with Plato, who qualifies this ability to reason by saying that it is subject to the will of his gods and one's desire to learn. The challenges to religious and political authorities that occurred between the 15th and 18th Centuries were based on a profound trust in mankind's intellect. But trusting Reason is problematic. Man's ability to reason is limited by his ability to perceive. The most-ignored truth of the Second Millenium has been that mankind cannot perceive all things.
Reason is a tool, particularly handy in science and research.
However, there is an entire realm of our existence that is ignored if we make Reason our mistress. And that is the realm of the Spirit. Most Christians who lived before the days of John Calvin recognized the fact that we cannot percieve God on our own. He has revealed Himself to us as Father, Son and Holy Spirit. It is up to us to approach Him on these terms.