Two things are requisite for faith. First, that the things which are of faith should be proposed to man: this is necessary in order that man believe anything explicitly. The second thing requisite for faith is the assent of the believer to the things which are proposed to him. Accordingly, as regards the first of these, faith must needs be from God. Because those things which are of faith surpass human reason, hence they do not come to man’s knowledge, unless God reveal them. To some, indeed, they are revealed by God immediately, as those things which were revealed to the apostles and prophets, while to some they are proposed by God in sending preachers of the faith, according to Rm. 10:15: “How shall they preach, unless they be sent?”

As regards the second, viz. man’s assent to the things which are of faith, we may observe a twofold cause, one of external inducement, such as seeing a miracle, or being persuaded by someone to embrace the faith: neither of which is a sufficient cause, since of those who see the same miracle, or who hear the same sermon, some believe, and some do not. Hence we must assert another internal cause, which moves man inwardly to assent to matters of faith.

The Pelagians held that this cause was nothing else than man’s free-will: and consequently they said that the beginning of faith is from ourselves, inasmuch as, to wit, it is in our power to be ready to assent to things which are of faith, but that the consummation of faith is from God, Who proposes to us the things we have to believe. But this is false, for, since man, by assenting to matters of faith, is raised above his nature, this must needs accrue to him from some supernatural principle moving him inwardly; and this is God. Therefore faith, as regards the assent which is the chief act of faith, is from God moving man inwardly by grace.

ST II-II near the beginning