What we mean by the word "faith".

The word through Anglo-Norman (fed) or Old French (feid, feit) from the Latin fides.
In the SOED there are 7 senses for the word "faith"used as a noun:

I - Confidence, belief.

1. (a) Confidence, reliance, belief esp. without evidence. (We have lost faith in government.)
(b) Belief founded upon testimony or authority. (The absolute rejection of authority...the annihilation of the spirit of blind faith.)
2.What is or should be believed; a system of firmly held beliefs or principles; a religion. (She abandoned her faith)
3. (a) Belief in the doctrines of a religion. esp such as affect character and conduct.
(b) The spiritual apprehension of divine truth or intangible realities. (Faith is the faculty by which we realize unseen things.)
4. (a) The power to convince; authority, credibility.
(b) Attestation, confirmation, assurance.

II - Fidelity

5. A pledge, a solemn promise. (I give to you my faith and truth... I will marry you)
6. The duty of fulfilling a trust or a promise; allegiance, obligation. (The Lords took their oaths of faith and allegiance.)
7. The fulfillment of a trust or promise; fidelity, loyalty. (Person's of great faith to his Majesty's cause.)