How rationalism runs faith aground is easy enough to understand. If the only admissible religious beliefs are those which can be established or otherwise secured by human reason, then there is no room for the unmerited, freely accepted gift of divine faith. Nor is there any room for the object of such faith: divine revelation. For such revelation supplies us, among other things, with truths that cannot be established or otherwise secured by natural reason; but ruling out divine faith entails ruling out recognizing, and trusting, any religious authority as the conveyor of divine revelation. Religion thus reduces to a matter of opinion—for there are no methods of proof that even religionists agree on. That was pretty much the view of Thomas Jefferson and of many others influenced by the Enlightenment. It is still the view of many philosophers today.
At the same time, fideism has undergone something of a renaissance among unbelievers as well as believers. Fideism is the view that religion, and with it divine faith, has nothing to do with reason, in the sense that "reasons" for having faith are as unnecessary as they are insufficient. The most common reason some believers end up as fideists is their conviction that the ultimate object of divine faith, God, so far transcends our reasoning capacities that rational criteria cannot be used to assess beliefs about God. Such beliefs are thus seen as insulated from rational criticism, which in its turn is seen as either ignorance or blasphemy. Most Sunni Muslims, and not a few Protestant and Orthodox Christians, are fideists in such a way. But to those of a scientific or otherwise critical bent, fideism can itself count as a reason against religious belief. For if religion is held to have nothing to do with reason, so that no particular religious belief can or should be subjected to rational scrutiny, then religion itself appears as positively irrational. And a cognitive stance that's irrational is not worthy of rational credence. Thus can rationalism and fideism, like so many pairs of extreme ideas, converge in similar attitudes.
To me, it seems almost self-evident that fideism, every bit as much as rationalism, is incompatible with divine faith. If rational criteria are not applicable to religious beliefs, then the fideist can only see the content of such beliefs as established arbitrarily by the absolute will of God. But such radical voluntarism empties the transcendental concepts of truth and goodness of all intrinsic content, and thus of all intrinsic meaning. It reduces religion to the blind worship of power. Some may find that prudent, but it hardly even makes sense to call it admirable.
The only way to steer between Scylla and Charybdis here is to insist, like such thinkers as Thomas Aquinas, both that divine faith cannot be established by human reason and that such faith must all the same be a reasonable choice. "Reasons", called "motives of credibility" in traditional apologetics, thus render divine faith rationally intelligible but not rationally necessitated. Such faith matches creation, for creation itself is rationally intelligible yet not rationally necessitated. Both faith and creation are thus mysteries in a positive sense—mysteries that will never be entirely dissipated when we see God face to face.
