In Christian theology, fideism is any of a number of positions.

It is occasionally used to refer to a belief that Christians are saved by faith alone: for which see sola fide. This position is sometimes called solifidianism. Note that sola fide is a doctrine mostly accepted by Protestants.

A more widely used meaning for the term is that fideism essentially teaches that reason is more-or-less irrelevant to religious belief. Specifically, fideism teaches that arguments for the existence of God are fallacious and irrelevant, and have nothing to do with the truth of Christian theology. Its argument in essence goes:

• Christian theology teaches that people are saved by faith.
• But, if God's existence can be proven, either empirically or logically, faith becomes irrelevant.
• Therefore, if Christian theology is true, no proof of God's existence is possible.

This sort of fideism has a long history in Christianity. It can plausibly be argued as an interpretation of 1 Corinthians, wherein Paul says, "For since, in the wisdom of God, the world did not know God through wisdom, it pleased God through the folly of what we preach to save those who believe . . . For the foolishness of God is wiser than (the wisdom of) men" (1 Cor. 1:21, 25) The statement "Credo quia absurdum" ("I believe because it is absurd"), often attributed to Tertullian, is sometimes cited as an example of such a view in the Church Fathers, but this appears to be a misquotation from Tertullian's De Carne Christi (External Link: On the Flesh of Christ). What he actually says in DCC 5 is ". . . the Son of God died; it is by all means to be believed, because it is absurd." This may be a statement of a fideist position, but it is also possible--and rendered somewhat plausible by the context--that Tertullian was simply engaging in ironic overstatement. A fideist position of this general sort -- that God's existence cannot be certainly known, and that the decision to accept faith is neither founded on, nor needs, rational justification --- may be found in the writings of Søren Kierkegaard (though his writings are notoriously hard to interpret) and his followers in Christian existentialism.

A more sophisticated form of fideism is assumed by Pascal's Wager. Blaise Pascal invites the sceptic to see faith in God as a cost-free choice that carries a potential reward. He does not attempt to argue that God indeed exists, only that it might be valuable to assume that it is true. Pascal's attitude has some commonality with another prominent Catholic writer of his period, Michel de Montaigne, who in his Essays shows a certain amount of sympathy with skepticism.

Presuppositional apologetics is a Christian system of apologetics associated with Calvinism; it attempts to distinguish itself from fideism, although some may find the difference elusive. It holds that Christian theology must begin with the proposition that the revelations contained in the Bible (or any other sources of divine revelation) are axiomatic. To a non-believer who rejects the notion that truth about God can be found within those sources, Christian theology literally has nothing to say.

Some theologies, however, strongly reject fideism. The Catechism of the Catholic Church, representing Roman Catholicism's great regard for Thomism, the teachings of St Thomas Aquinas, affirms that it is a doctrine of Roman Catholicism that God's existence can indeed be demonstrated by reason. Likewise, a tradition of argument found among some Protestant fundamentalists as well as Catholics argues that respect for Jesus as a teacher and a wise man is logically contradictory if one does not accept him as God as well, also known as the Lord, Liar, or Lunatic argument: either He was insane or a charlatan, or he was in fact the Messiah and Son of God. Cf., Christological argument

While the centrality of issues of faith and its role in salvation make fideism of this sort an important issue for Christianity, it can exist in other revealed religions as well. In Islam, the theologian Ghazali strikes a position similar to Tertullian's fideism in his Talafut al-falasafa, the "Incoherence of the Philosophers." Where the claims of reason come into conflict with revelation, reason must yield to revelation. This position drew a rejoinder from Averroes, whose position was more influential in Thomist and other medieval Christian thinking than it was in the Islamic world itself. Ghazali's position of the absolute authority and finality of divine revelation became the standard of orthodox Muslim exegesis.