Faith is a belief in the trustworthiness of an idea. Formal usage of the word "faith" is usually reserved for concepts of religion, as in theology, where it almost universally refers to a trusting belief in a transcendent reality, or else in a Supreme Being and said being's role in the order of transcendent, spiritual things.

Informal usage of the word "faith" can be quite broad, and may be used standardly in place of either as "trust," "belief," or "hope". For example, the word "faith" can refer to a religion itself or to religion in general. (For informal uses of the word "faith", see Faith (word)). As with "trust," faith involves a concept of future events or outcomes.

Epistemological validity of faith

There exists a wide spectrum of opinion with respect to the epistemological validity of faith. On one extreme is logical positivism, which denies the validity of any beliefs held by faith; on the other extreme is fideism, which holds that true belief can only arise from faith, because reason and evidence cannot lead to truth. Some foundationalists, such as St. Augustine of Hippo and Alvin Plantinga, hold that all of our beliefs rest ultimately on beliefs accepted by faith. Others, such as C. S. Lewis, hold that faith is merely the virtue by which we hold to our reasoned ideas, despite moods to the contrary.

Fideism and Pistisism

Fideism is not a synonym for “religious belief,” but describes a particular philosophical proposition in regard to the relationship between faith's appropriate jurisdiction at arriving at truths, contrasted against reasons. It states that faith is needed to determine some philosophical and religious truths, and it questions the ability of reason to arrive at all truth. The word and concept had its origin in the mid to late nineteenth century by way of Roman Catholic thought, in a movement called traditionalism. The Roman Catholic Magisterium has repeatedly condemned fideism though.

The word is also occasionally used to refer to the Protestant belief that Christians are saved by faith alone: for which see sola fide. This position is sometimes called solifidianism and sol Pistisism.

Many noted philosophers and theologians have espoused the idea that faith is the basis of knowledge. One example is St. Augustine of Hippo. Known as one of his contributions to philosophy, the idea of "faith seeking understanding" was set forth by St. Augustine in his statement "Crede, ut intelligas" ("Believe in order that you may understand").

One illustration of this concept is in the development of knowledge in children. A child typically holds parental teaching as credible, in spite of the child's lack of sufficient research to establish such credibility empirically. That parental teaching, however fallible, becomes a foundation upon which future knowledge is built.Fact: date=February 2007 The child's faith in his/her parents teaching is based on a belief in their credibility. Unless/until the child's belief in their parents' credibility is superseded by a stronger belief, the parental teaching will serve as a filter through which other teaching must be processed and/or evaluated. Following this line of reasoning, and assuming that children have finite or limited empirical knowledge at birth, it follows that faith is the fundamental basis of all knowledge one has. Even adults attribute the basis for some of their knowledge to so called "authorities" in a given field of study. This is true because one simply does not have the time or resources to evaluate all of his/her knowledge empirically and exhaustively. "Faith" is used instead.