mid-13c., faith, feith, fei, fai "faithfulness to a trust or promise; loyalty to a person; honesty, truthfulness," from Anglo-French and Old French feid, foi "faith, belief, trust, confidence; pledge" (11c.), from Latin fides "trust, faith, confidence, reliance, credence, belief," from root of fidere "to trust,"from PIE root *bheidh- "to trust, confide, persuade." For sense evolution, see belief. Accommodated to other English abstract nouns in -th (truth, health, etc.).
From early 14c. as "assent of the mind to the truth of a statement for which there is incomplete evidence," especially "belief in religious matters" (matched with hope and charity). Since mid-14c. in reference to the Christian church or religion; from late 14c. in reference to any religious persuasion.
And faith is neither the submission of the reason, nor is it the acceptance, simply and absolutely upon testimony, of what reason cannot reach. Faith is: the being able to cleave to a power of goodness appealing to our higher and real self, not to our lower and apparent self. [Matthew Arnold, "Literature & Dogma," 1873]
From late 14c. as "confidence in a person or thing with reference to truthfulness or reliability," also "fidelity of one spouse to another." Also in Middle English "a sworn oath," hence its frequent use in Middle English oaths and asseverations (par ma fay, mid-13c.; bi my fay, c. 1300).
The power which a man's imagination has over his body to heal it or make it sick is a force which none of us is born without. The first man had it, the last one will possess it. If left to himself, a man is most likely to use only the mischievous half of the force—the half which invents imaginary ailments for him and cultivates them; and if he is one of those very wise people, he is quite likely to scoff at the beneficent half of the force and deny its existence. ["Mark Twain," "Christian Science," 1907]
Unfaithful ... especially means a lack of fidelity to trust or duty, a failure to perform what is due, however much may be implied in that. Faithless is negative in form, but positive in sense; the faithless man does something which is a breach of faith; the sleeping sentinel is unfaithful; the deserter is faithless. [Century Dictionary, 1895]
early 14c., "sincerely religious, devout, pious," especially in reference to Christian practice; mid-14c., "loyal (to a lord, friend, spouse, etc.); true; honest, trustworthy," from faith + -ful. From late 14c. in reference to a tale, a report, etc., "accurate, reliable, true to the facts." The noun sense of "true believer, one who is full of faith" is from late 14c. (Church Latin used fideles in same sense). Related: Faithfully; faithfulness. Old Faithful geyser named 1870 by explorer Gen. Henry Dana Washburn, surveyor-general of the Montana Territory, in reference to the regularity of its outbursts.
late 12c., bileave, "confidence reposed in a person or thing; faith in a religion," replacing Old English geleafa "belief, faith," from West Germanic *ga-laubon "to hold dear, esteem, trust" (source also of Old Saxon gilobo, Middle Dutch gelove, Old High German giloubo, German Glaube), from *galaub- "dear, esteemed," from intensive prefix *ga- + PIE root *leubh- "to care, desire, love." The prefix was altered on analogy of the verb believe. The distinction of the final consonant from that of believe developed 15c.
The be-, which is not a natural prefix of nouns, was prefixed on the analogy of the vb. (where it is naturally an intensive) .... [OED]
Meaning "conviction of the truth of a proposition or alleged fact without knowledge" is by 1530s; it is also "sometimes used to include the absolute conviction or certainty which accompanies knowledge" [Century Dictionary]. From c. 1200 as "a creed, essential doctrines of a religion or church, things held to be true as a matter of religious doctrine;" the general sense of "That which is believed" is by 1714. Related: Beliefs.
Belief meant "trust in God," while faith meant "loyalty to a person based on promise or duty" (a sense preserved in keep one's faith, in good (or bad) faith, and in common usage of faithful, faithless, which contain no notion of divinity). But faith, as cognate of Latin fides, took on the religious sense beginning in 14c. translations, and belief had by 16c. become limited to "mental acceptance of something as true," from the religious use in the sense of "things held to be true as a matter of religious doctrine."