The Enlightenment thinkers felt that reason had come to age and elevated it above faith. But, mathematics is not the book of life. It is impossible to expel faith. As the wise sage Pascal observed, reason has to be find support on something, and that something can't be reason itself, to avoid circular reasoning. Reason is also based upon faith. Again, he showed that reason cannot irrefutably prove God, but it can neither disprove His existence. It proves nothing.
Spiritual skepticism is the willingness to concede that our minds cannot have certainty alone by themselves. This is not to deny certain certainties that are axiomatic and undeniable. For instance, one can't contradict the law of non-contradiction. However, it certainly means that one has no confidence in his own reasoning and wisdom. Only this form of skepticism regarding self can truly set us free to find our fullness and certainty only in Him.