"For I know thy rebellion, and thy stiff neck: behold, while I am yet alive with you this day, ye have been rebellious against the Lord; and how much more after my death?" (Deut. 31: 27)
The word faith is surprisingly difficult to define: it can mean acceptance of a set of beliefs, it can be used to express an attitude of confidence and trust. To be "faithful" implies consistency and fidelity, talking about a "faith" denotes a set of beliefs and an accompanying community of relationships. Why did God choose such a slippery word to instruct us?
We try to anchor ourselves to the word, at times, by speaking of its opposite.
If the opposite of faith is doubt, faith is primarily an intellectual principle, practically synonymous with belief (not an unslippery word itself, as Mark 9: 24 shows).
If the opposite of faith is fear, then faith is primarily an emotional principle, trust made courage--but what, then, of the scripturally desirable fear of the Lord?
Perhaps it is better to see rebellion as faith's opposite. Faith, then, is a more a matter of how we choose to align ourselves than about the more passive elements of belief. Faith is less a matter of head or heart than of feet, and where they stand.
Or maybe it's best to see faith as a principle of many opposites, a word God made slippery precisely so that it can face us in every aspect of self.