Objection 1. It would seem that to believe in not meritorious. For the principle of all merit is charity, as stated above (I-II, 114, 4). Now faith, like nature, is a preamble to charity. Therefore, just as an act of nature is not meritorious, since we do not merit by our natural gifts, so neither is an act of faith.
Objection 2. Further, belief is a mean between opinion and scientific knowledge or the consideration of things scientifically known [Science is a certain knowledge of a demonstrated conclusion through its demonstration.]. Now the considerations of science are not meritorious, nor on the other hand is opinion. Therefore belief is not meritorious.
Objection 3. Further, he who assents to a point of faith, either has a sufficient motive for believing, or he has not. If he has a sufficient motive for his belief, this does not seem to imply any merit on his part, since he is no longer free to believe or not to believe: whereas if he has not a sufficient motive for believing, this is a mark of levity, according to Sirach 19:4: "He that is hasty to give credit, is light of heart," so that, seemingly, he gains no merit thereby. Therefore to believe is by no means meritorious.
On the contrary, It is written (Hebrews 11:33) that the saints "by faith... obtained promises," which would not be the case if they did not merit by believing. Therefore to believe is meritorious.
I answer that, As stated above (I-II, 114, 3,4), our actions are meritorious in so far as they proceed from the free-will moved with grace by God. Therefore every human act proceeding from the free-will, if it be referred to God, can be meritorious. Now the act of believing is an act of the intellect assenting to the Divine truth at the command of the will moved by the grace of God, so that it is subject to the free-will in relation to God; and consequently the act of faith can be meritorious.
Reply to Objection 1. Nature is compared to charity which is the principle of merit, as matter to form: whereas faith is compared to charity as the disposition which precedes the ultimate form. Now it is evident that the subject or the matter cannot act save by virtue of the form, nor can a preceding disposition, before the advent of the form: but after the advent of the form, both the subject and the preceding disposition act by virtue of the form, which is the chief principle of action, even as the heat of fire acts by virtue of the substantial form of fire. Accordingly neither nature nor faith can, without charity, produce a meritorious act; but, when accompanied by charity, the act of faith is made meritorious thereby, even as an act of nature, and a natural act of the free-will.
Reply to Objection 2. Two things may be considered in science: namely the scientist's assent to a scientific fact and his consideration of that fact. Now the assent of science is not subject to free-will, because the scientist is obliged to assent by force of the demonstration, wherefore scientific assent is not meritorious. But the actual consideration of what a man knows scientifically is subject to his free-will, for it is in his power to consider or not to consider. Hence scientific consideration may be meritorious if it be referred to the end of charity, i.e. to the honor of God or the good of our neighbor. On the other hand, in the case of faith, both these things are subject to the free-will so that in both respects the act of faith can be meritorious: whereas in the case of opinion, there is no firm assent, since it is weak and infirm, as the Philosopher observes (Poster. i, 33), so that it does not seem to proceed from a perfect act of the will: and for this reason, as regards the assent, it does not appear to be very meritorious, though it can be as regards the actual consideration.
Reply to Objection 3. The believer has sufficient motive for believing, for he is moved by the authority of Divine teaching confirmed by miracles, and, what is more, by the inward instinct of the Divine invitation: hence he does not believe lightly. He has not, however, sufficient reason for scientific knowledge, hence he does not lose the merit.
Source: Summa Theologica (New Advent)