Fideism (/ˈfiːdeɪ.ɪzəm, ˈfaɪdiː-/ FEE-day-iz-əm, FY-dee-) is a standpoint or an epistemological theory which maintains that faith is independent of reason, or that reason and faith are hostile to each other and faith is superior at arriving at particular truths (see natural theology). The word fideism comes from fides, the Latin word for faith, and literally means "faith-ism". Philosophers have identified a number of different forms of fideism. Strict fideists hold that reason has no place in discovering theological truths, while moderate fideists hold that though some truth can be known by reason, faith stands above reason.

Theologians and philosophers have responded in various ways to the place of faith and reason in determining the truth of metaphysical ideas, morality, and religious beliefs. Historically, fideism is most commonly ascribed to four philosophers: Søren Kierkegaard, Blaise Pascal, William James, and Ludwig Wittgenstein; with fideism being a label applied in a negative sense by their opponents, but which is not always supported by their own ideas and works or followers. A qualified form of fideism is sometimes attributed to Immanuel Kant's famous suggestion that we must "deny knowledge in order to make room for faith".

Alvin Plantinga defines "fideism" as "the exclusive or basic reliance upon faith alone, accompanied by a consequent disparagement of reason and is used especially in the pursuit of philosophical or religious truth". The fideist therefore "urges reliance on faith rather than reason, in matters philosophical and religious", and therefore may go on to disparage the claims of reason. The fideist seeks truth, above all, and affirms that reason cannot achieve certain kinds of truth, which must instead be accepted only by faith.

The doctrine of fideism is consistent with some, and radically contrary to other theories of truth:

Tertullian taught fideistic concepts such as the later philosophers William of Ockham and Søren Kierkegaard. Tertullian's De Carne Christi (On the Flesh of Christ]) says "the Son of God died; it is by all means to be believed, because it is absurd."

On the other hand, some deny Tertullian's fideistic character, the statement "Credo quia absurdum" ("I believe because it is absurd") is sometimes cited as an example of views of the Church Fathers. However, this has been argued to have been a misquotation of Tertullian, saying that Tertullian was critiquing intellectual arrogance and the misuse of philosophy, but that he remained committed to reason and its usefulness in defending the faith.

Ockham was a fideist, holding that belief in God is only a matter of faith and not from knowledge; this led him to deny all the alleged proofs of God.

Martin Luther taught that faith informs the Christian's use of reason. Regarding the mysteries of Christian faith, he wrote, "All the articles of our Christian faith, which God has revealed to us in His Word, are in presence of reason sheerly impossible, absurd, and false." And "Reason is the greatest enemy that faith has." However, Luther conceded that, grounded upon faith in Christ, reason can be used in its proper realm, as he wrote, "Before faith and the knowledge of God reason is darkness in divine matters, but through faith it is turned into a light in the believer and serves piety as an excellent instrument. For just as all natural endowments serve to further impiety in the godless, so they serve to further salvation in the godly. An eloquent tongue promotes faith; reason makes speech clear, and everything helps faith forward. Reason receives life from faith; it is killed by it and brought back to life."