Fideism is a name often given to a particular approach to religious epistemology which holds that all religious and philosophical knowledge is impossible to prove and therefore must be believed by faith. In fact, the name "fideism" comes from the Latin word fides, which means "faith." Fideism is therefore opposed to other perspectives of religious epistemology, including rationalism, the belief that religious knowledge is all and only what can be conclusively proved on the basis of first principles, and skepticism, the belief that no religious knowledge can be conclusively proved, so that no religious truth claims are worthy of belief.
Fideism - What is it?
The fideistic perspective differs from the rationalistic perspective in that, while both would accept certain truth claims set forward by, say, Christian theism, they would disagree as to whether (and how) those truth claims were to be justified. For example, consider a very basic truth claim of Christianity, namely, that God exists. A rationalist might argue that God must exist by appealing to any one of the classic arguments in support of this claim. On the other hand, a fideist might respond by arguing that God can only be known by faith, by taking the teachings of the Bible at face value. In this sense, both the fideist and the rationalist agree that God exists, but they disagree as to how we know that God exists.
Similarly, the fideistic perspective is different from the skeptical perspective in important ways. However, in this case, the two perspectives agree that the claim "God exists" cannot be conclusively proved, but they disagree as to whether it ought to be believed. In other words, they agree that the rationalist approach to proving God's existence fails, but they disagree as to how to handle this failure. The fideist argues that, even though we cannot prove God exists, we should still believe by faith, whereas the skeptic argues that, since we cannot prove God exists, we therefore should not believe that God exists at all.
Is fideism biblical? It depends on how it is used. On the one hand, fideism is certainly a biblical way to approach one's relationship with God; after all, Hebrews 11:6 teaches that "without faith it is impossible to please him [God], for whoever would draw near to God must believe that he exists and that he rewards those who seek him." On the other hand, fideism is not, in general, a biblical way of defending one's faith or of showing it to be true: even Jesus demonstrated the reality of His own resurrection to His disciples by "many proofs" (Acts 1:3), without expecting them to "just believe" without any evidence whatsoever. In short, faith is the attitude with which we approach God, not how we justify belief in His existence.
This helps us to understand why there is so much confusion about whether it is faith or reason which ought to ground (i.e., justify) the beliefs of the Christian: many wrongly assume, in a fideistic fashion, that God demands faith, even at the expense of using reason. However, this is simply a confusion of categories: there is nothing unbiblical about using reason to justify belief in God's existence, and faith to respond to that belief in a biblically appropriate way. Christians get into problems, on the one hand, when they try to justify belief in God by relying on faith (this is the mistake made by misapplying fideism), or on the other hand, when they refuse to believe and obey God's commands without first receiving "proof" that He exists and wants them to live in a particular way. The distinction between how one justifies Christian belief and the attitude with which one responds to that belief is essential to relying upon faith in a biblical manner.
Ultimately, biblical faith is very different from mere "blind faith." Biblical faith has less to do with justifying the truth claims of Scripture than with ensuring that we have the proper response once we recognize these truth claims to be true. Fideism is thus only appropriate as a biblical response to Christian truth, and not an adequate justification of it.
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