Well, it gets kind of complicated there too, but it all boils down to belief, if you have something you believe in, that is where the Faith (capital f intentional) comes from. Like for example, Dresden has Faith in magic, and that can give him some powers right there, whereas his best friend Michael, truly believes everything in the Bible, and that the sword he owns contains one of the nails from the Cross of the Crucifitcion (one of three) and the powers of faith he gets form that helps him to be one of the Knights of the Cross (serious badasses, though they never use magic, their faith allows them to take on some of the scariest things out there)