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 remains 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).
Latin
I. FIDES Theologice sumpta in genere, est assensus quem Deo loquenti praebemus, quum aliquid verum esse certo nobis persuademus, ad id Divina Authoritate moti. Ac per hoc fides distinguitur ab opinione, quae assensus est timidus et incertus. Item a scientia, quae quidem assensus est certus, sed qui demonstratione, aut evidenti aliquo argumento, non vero cujusquam testimonio nititur. Denique a fide humana, quae in authoritate hominum acquiescit. II. Itaque quivis assensus, adhibitus etiam eis, quae verbo Dei continentur, fides non est censendus. Si quis enim aliquod religionis dogma, veluti universorum creationem ex nihilo, certo et firmiter teneat: non tamen ad id impulsus revelatione divina, sed ratione quadam, quae ipsi necessaria videtur, eatenus fidem non habet. Ut nec qui Israelitici populi historiam credit ex relatione Josephi Historici, non vero historicis sacris fidem adhibens. Quod et de iis dici potest, qui verae et orthodoxae religioni adhaerescunt, sed tantum ex consuetudine, et ad id sola majorum authoritate moti. III. Fidei vero varias, sive species, sive gradus, ex Scripturis assignare licet. In quibusdam enim fides intellectu theoretico haeret: et ita nudum et simplicem assensum praebent iis, quae verbo Dei proponuntur, ut ad recte agendum inde non moveantur, nec reddantur meliores. Ita credidit Simon Magus praedicationi Philippi, cujus tamen cor non erat rectum, quique in amarissimo felle, et nexu injustitiae positus erat, ut habetur Actorum 8. Ita quoque crediderant in nomen Christi, illi quibus tamen Christus non credebat seipsum, quod probe nosset omnes, prout legimus, Johan. 2. Et hanc suo modo participant ipsi daemones, qui dicuntur apud Jacobum credere et contremiscere.