Faith is assent to God speaking, moved by divine authority — and not every assent counts as faith

Faith, taken theologically in general, is the assent we give to God speaking, when we are certainly persuaded that something is true, moved to it by divine authority. By this, faith is distinguished from opinion, which is a timid and uncertain assent. Likewise from science, which is indeed a certain assent, but one that rests on demonstration or some evident argument, not on anyone's testimony. Finally from human faith, which rests in the authority of men.

Therefore not every assent, even when given to things contained in the word of God, is to be counted as faith. For if someone holds some doctrine of religion with certainty and firmness — for example the creation of all things out of nothing — yet is not driven to it by divine revelation, but by some reason that seems necessary to him, to that extent he does not have faith. Nor does the one who believes the history of the Israelite people from the report of Josephus the historian, and does not give faith to the sacred historians. The same can be said of those who cling to the true and orthodox religion, but only from custom, and moved to it by the authority of their ancestors alone.

But various species or grades of faith may be assigned from the Scriptures. In some people faith sticks in the theoretical intellect alone: and so they give a bare and simple assent to what is proposed in the word of God, yet are not moved thereby to act rightly, nor are they made better. So Simon Magus believed Philip's preaching, yet his heart was not right, and he was in the bitterest gall and the bond of injustice, as is recorded in Acts 8. So also those had believed in the name of Christ to whom nevertheless Christ did not entrust himself, because he knew all people thoroughly, as we read in John 2. And even the demons themselves share in this in their own way, who are said in James to believe and tremble (James 2:19).