Objection 1. It would seem that faith is not infused into man by God. For Augustine says (De Trin. xiv) that "science begets faith in us, and nourishes, defends and strengthens it." Now those things which science begets in us seem to be acquired rather than infused. Therefore faith does not seem to be in us by Divine infusion.
Objection 2. Further, that to which man attains by hearing and seeing, seems to be acquired by him. Now man attains to belief, both by seeing miracles, and by hearing the teachings of faith: for it is written (John 4:53): "The father... knew that it was at the same hour, that Jesus said to him, Thy son liveth; and himself believed, and his whole house"; and (Romans 10:17) it is said that "faith is through hearing." Therefore man attains to faith by acquiring it.
Objection 3. Further, that which depends on a man's will can be acquired by him. But "faith depends on the believer's will," according to Augustine (De Praedest. Sanct. v). Therefore faith can be acquired by man.
On the contrary, It is written (Ephesians 2:8-9): "By grace you are saved through faith, and that not of yourselves... that no man may glory... for it is the gift of God."
I answer that, 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 Romans 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.
Reply to Objection 1. Science begets and nourishes faith, by way of external persuasion afforded by science; but the chief and proper cause of faith is that which moves man inwardly to assent.
Reply to Objection 2. This argument again refers to the cause that proposes outwardly the things that are of faith, or persuades man to believe by words or deeds.
Reply to Objection 3. To believe does indeed depend on the will of the believer: but man's will needs to be prepared by God with grace, in order that he may be raised to things which are above his nature, as stated above (2, 3).
Source: Summa Theologica (New Advent)