Faith is simply the conviction that life is meaning-full.
This passage about faith in Introduction to Christianity is the same passage were Ratzinger describes what he calls "the primacy of the invisible." I use this phrase as the title of one of the four new chapters in the paperback edition of Hunting Magic Eels. Here is Ratzinger describing this:
Christian belief--as we have already said--means opting for the view that what cannot be seen is more real than what can be seen. It is an avowal of the primacy of the invisible as the truly real, which upholds us and hence enables us to face the visible with calm composure--knowing that we are responsible before the invisible as the true ground of all things.
As I describe in the new chapter of Hunting Magic Eels, what is most primary about your life is invisible. For your valuation of the world, how you imbue life with meaning, cannot be detected by any scientific instrument or empirical measuring devise. And yet, it is the primacy of the invisible which will push and pull you more strongly than the law of gravity.
To be sure, we can measure the behavioral, psychological, and sociological effects of value. They are gravity wells we orbit. But at the center of the orbit, like a black hole, value itself cannot be empirically measured or observed. Value only tugs upon subjective human consciousness which, by definition, falls beyond third-person, empirical observation.
Value, the most important causal influence upon your life, is both invisible and primary.