What is faith? Is it something irrational? No. Something unprovable? No, again. Is it something that makes us religious? Not necessarily. Faith is will. It's that simple.
Will is not irrational. Moreover, it's not even rational, but rather is the basis of that which is rational. We posit rationality: a set of premises from which we reason according to a set of rules. We posit concepts such as non-contradiction and uniformity. We will them, despite their fallibility, and we shape our knowledge around them.
Will is not unprovable. We prove it through action that leads to experience and knowledge. We posit that they can love us, act on that position and subsequently learn of their love or lack thereof.
Will is not necessarily religious. It is creation and discovery generally, without limitation to traditionally religious ends. Yet, of course, there is the religious aspect of faith. The theist claims faith in God. Although certainly not all theists use "faith" as I have described it here, that faith is nonetheless most tangible and potent when understood as will in God. As will, faith in God does not depend on any particular existential claims, except the existence of the faith itself: discover and join God to the extent God now exists; create and become God to the extent God does not now exist.
There are many Gods, and not all are equal. Faith in one may prove fruitless or worse, whereas faith in another may prove fulfilling. As with all acts of will, faith in God is proven in experience, leading us to a knowledge of the posited God, whether it be worthy of worship or otherwise. The golden calf, stone giant and metaphysical abstraction may prove relatively impotent, despite our will to them. There may be, however, a God or Gods that prove themselves to our satisfaction.
What is the object of your will? What is most worth discovering and creating? That is your faith in God, and it will be proven one way or another.
[Thanks for reading! You might also like "Dynamic Faith in Pancritical Rationalism".]