Origins
Faith entered English from Old French in the 13th century, but the concept it names goes back to the earliest human societies. The Latin fidēs meant 'trust' in the broadest sense — trust between people, trust in gods, trust as the foundation of treaties.
The Proto-Indo-European root *bheidh- meant simply 'to trust' or 'to confide'. From this single root grew an astonishing family of English words: fidelity (loyal trust), confide (share trust), federal (bound by trust), fiduciary (held in trust), and infidel (without trust).
Latin Roots
The religious meaning — belief without empirical evidence — developed in theological Latin. But the older, secular sense of faith as interpersonal trust remains alive in phrases like 'good faith', 'faith in someone', and 'faithful friend'.
What makes this etymology revealing is how it connects religion, finance, and politics through a single concept. A federal government, a fiduciary duty, and religious faith are all, at root, the same word — all built on the ancient human need to trust.