In fact, doing a rudimentary search through the bible, I found that in the Hebrew Bible, the Hebrew words for faith and trust (aman and batach) appear 382 times. In the New Testament pistis (noun, "faith") and pisteuo (verb "have faith" or "believe"), are found in 444 verses. That is a fairly big number, as should be expected.
The question that remains though is what exactly IS faith? Even the disciples seem to misunderstand when they ask for their faith to be increased... as if it were some sort of magical commodity.
I turned today then to Melanchthon's Apology of the Augsburg Confession where he talks about faith and in particular justifying faith. He writes:
But that faith which justifies is not merely a knowledge of history, [not merely this, that I know the stories of Christ's birth, suffering, etc. (that even the devils know,)] but it is to assent to the promise of God, in which, for Christ's sake, the remission of sins and justification are freely offered. [It is the certainty or the certain trust in the heart, when, with my whole heart, I regard the promises of God as certain and true, through which there are offered me, without my merit, the forgiveness of sins, grace, and all salvation, through Christ the Mediator.] And that no one may suppose that it is mere knowledge, we will add further: it is to wish and to receive the offered promise of the remission of sins and of justification. [Faith is that my whole heart takes to itself this treasure. It is not my doing, not my presenting or giving, not my work or preparation, but that a heart comforts itself, and is perfectly confident with respect to this, namely, that God makes a present and gift to us, and not we to Him, that He sheds upon us every treasure of grace in Christ.]
49] And the difference between this faith and the righteousness of the Law can be easily discerned. Faith is the latreiva [divine service], which receives the benefits offered by God; the righteousness of the Law is the latreiva [divine service] which offers to God our merits. By faith God wishes to be worshiped in this way, that we receive from Him those things which He promises and offers.
50] Now, that faith signifies, not only a knowledge of the history, but such faith as assents to the promise, Paul plainly testifies when he says, Rom. 4:16: Therefore it is of faith, to the end the promise might be sure. For he judges that the promise cannot be received unless by faith. Wherefore he puts them together as things that belong to one another, and connects promise and faith. [There Paul fastens and binds together these two, thus: Wherever there is a promise faith is required, and conversely, wherever faith is required, there must be a promise.] 51] Although it will be easy to decide what faith is if we consider the Creed, where this article certainly stands: The forgiveness of sins. Therefore it is not enough to believe that Christ was born, suffered, was raised again, unless we add also this article, which is the purpose of the history: The forgiveness of sins. To this article the rest must be referred, namely, that for Christ's sake, and not for the sake of our merits, 52] forgiveness of sins is given us. For what need was there that Christ was given for our sins if for our sins our merits can make satisfaction?
53] As often, therefore, as we speak of justifying faith, we must keep in mind that these three objects concur: the promise, and that, too, gratuitous, and the merits of Christ, as the price and propitiation. The promise is received by faith; the "gratuitous" excludes our merits, and signifies that the benefit is offered only through mercy; the merits of Christ are the price, because there must be a certain propitiation for our sins. 54] Scripture frequently implores mercy; and the holy Fathers often say that we 55] are saved by mercy. As often, therefore, as mention is made of mercy, we must keep in mind that faith is there required, which receives the promise of mercy. And, again, as often as we speak of faith, we wish an object to be understood, namely, the promised mercy. 56] For faith justifies and saves, not on the ground that it is a work in itself worthy, but only because it receives the promised mercy. (from bookofconcord.org)
I am drawn to that phrase in the begining of this quote that faith is "to assent to the promise of God." I think this phrase is a good place to start our thinking about what faith is and is not.