Faith and the OED

Usually in English a monosyllabic word has a good chance of being a native English word, but faith is not. Although many French and Latin words were imported into English during the Hundred Year's War (AD 1337-1453), faith is actually brought in earlier. Here are the definitions of faith listed in the Oxford English Dictionary according to first usage:

1250 the duty of fulfilling one's trust, fealty, the obligation of a promise or engagement
1250 faithfulness, fidelity, loyalty
1300 confidence, reliance, trust
1300 the Christian faith
1325 a system of religious belief
1380 what is required to be believed on a subject
1382 assurance given, formal declaration, pledge, promise
1382 belief in the truths of religion
1393 attestation, confirmation, assurance
1551 belief preceding from reliance on testimony or authority
1638 power to produce belief

When the term faith entered the English language it meant loyalty (which the editors of the OED listed last). Later, it came to mean trust. After that time it came to mean a system of religious belief, about the same time when it came to mean a pledge or promise. Only much later did it come to be a belief based on something someone else said. (Ironically, the last meaning developed is listed as obsolete.)

So at first faith in God meant loyalty to God. A little later it came to mean trust in God. Only later did it weaken to belief in God. Far from being merely an intellectual assenting to the existence of God, faith in God was originally a loyalty to Him.