There is a short and sweet treatment of faith in the first Baltimore Catechism that throws into clear relief the difference between the Protestant and Catholic views of this virtue. It is found in question 321. First, remember that the Protestant view is a fiduciary one: it focuses upon trust, and specifically on trusting that one’s sins are forgiven by Christ plus nothing else. The Catholic rejoinder to that is that we must of course trust God, because God is infinitely trustworthy. So although the Church affirms the necessity of trusting God, faith is something quite different. The focus is on assent to the truth.
Q. How does a person sin against faith? A. A person sins against faith: 1st, by not trying to know what God has taught; 2d, by refusing to believe all that God has taught; 3d, by neglecting to profess his belief in what God has taught. (Baltimore Catechism I, question 321)
Although the BC states things negatively here, we may safely infer the obvious positives. Faith is belief in all that God has taught, with the concomitant duties of professing the faith when necessary and seeking to learn as best we can what it is that God has taught. The fundamental thing here is assenting to what God has said and/or taught, just because He said or taught it. There is an obvious presupposition as well, found in Hebrews 11:6:
Now it is impossible to please God without faith, since anyone who comes to him must believe that he exists and rewards those who seek him. (NJB; emphasis added)
This seems rather obvious to me, but it reflects the Catholic view of faith rather better than the Protestant one, I think. Here we get an eagle’s eye view of what faith is: in sum, the one who has faith believes that God exists and rewards those who seek Him. This is the presupposition to what the BC says. You can’t have faith in God if you do not believe that He exists! This is just a sine qua non that anyone would readily acknowledge.
As an aside, this verse also contradicts the Reformed Protestant notion of total depravity, according to which it is impossible for us to please God, full stop, and that God does reward anyone for anything because that implies a merit which they also claim is impossible for us to have. But I digress.
Returning to the BC, it says that it is a sin to not try to know what God has taught. Why? Because such negligence implies either that we do not care what God has said (which is obviously wrong) or that we do not care to know the truth at all (whether divinely revealed or otherwise). This too is evil, because God made us intellectual beings for a reason. We are made to love and to seek to know the truth. There are some things that we cannot know unless God tells them to us; consequently we ought to believe Him when He speaks and hold to the truth wherever we may find it because doing so is fundamental to what it means to be human.
Another aspect of the way the BC phrases things is that we must try to know what God has taught. This important qualifier means that poor people without the means or ability to know what God has taught are not liable for something that is outside their control. A contrasting example which helps make this clear, I think, is that the Protestant view removes any room for hope for people who are incapable of trusting God because they lack the mental capacity. But Catholics affirm the same hope for such people as for anyone else because God does not require from us what we are incapable of doing. So we try to know what God has taught us as much as our condition in life allows us to do so.
Lastly, we are obliged to believe all that God has taught. Why? Because refusing to believe it is to deny that what He said is true, and that is to call God a liar. But God cannot lie. He is truth. I am not free to pick and choose what I will believe among all that God has taught. For starters this would be wildly arrogant of me: to believe that I know the truth better than God does!
Faith is more than mere fiduciary trust according to the Baltimore Catechism. This is a distinct difference of emphasis from what our Protestant brethren profess.