Fideism
Fideism is the view that religious belief does not need — and sometimes cannot receive — rational justification, because faith is either prior to reason or operates on a different plane from it.
The term comes from the Latin fides, faith. A fideist holds that the central claims of religion are not the kind of thing that can be proved by argument or established by evidence, and that trying to do so either misunderstands what faith is or corrupts it. The strongest fideists (sometimes called radical fideists) go further: reason and faith are in principle opposed, and accepting a claim on faith is a virtue precisely because reason offers no support for it.
Historical figures associated with fideism include Tertullian ("I believe because it is absurd"), Søren Kierkegaard (who described faith as a "leap" across an "infinite qualitative difference" between God and humanity), and, in a softer form, Blaise Pascal. Kierkegaard in particular argued that religious truth is not the kind of truth that can be known at a distance like a geological fact. It has to be appropriated by the believer as a personal commitment, and the appropriation is what makes it faith rather than mere assent.
Critics of fideism, including most natural theologians and rationalist philosophers, argue that it is self-defeating. If belief in God doesn't need rational support, then neither does belief in Odin, or in crystal healing, or in any conspiracy theory. The believer needs some way to tell genuine faith from delusion, and any such criterion is itself a reason, which collapses fideism back into ordinary epistemology.
From a secular perspective, fideism is worth understanding because it is often the implicit framework of people who say that faith is "personal" and not open to argument. Recognizing the position by name clarifies the conversation and makes it possible to ask the fideist where the stopping rule is.