Objection 1. It would seem that reasons in support of what we believe lessen the merit of faith. For Gregory says (Hom. xxvi in Evang.) that "there is no merit in believing what is shown by reason." If, therefore, human reason provides sufficient proof, the merit of faith is altogether taken away. Therefore it seems that any kind of human reasoning in support of matters of faith, diminishes the merit of believing.
Objection 2. Further, whatever lessens the measure of virtue, lessens the amount of merit, since "happiness is the reward of virtue," as the Philosopher states (Ethic. i, 9). Now human reasoning seems to diminish the measure of the virtue of faith, since it is essential to faith to be about the unseen, as stated above (1, 4,5). Now the more a thing is supported by reasons the less is it unseen. Therefore human reasons in support of matters of faith diminish the merit of faith.
Objection 3. Further, contrary things have contrary causes. Now an inducement in opposition to faith increases the merit of faith whether it consist in persecution inflicted by one who endeavors to force a man to renounce his faith, or in an argument persuading him to do so. Therefore reasons in support of faith diminish the merit of faith.
On the contrary, It is written (1 Peter 3:15): "Being ready always to satisfy every one that asketh you a reason of that faith [Vulgate: 'Of that hope which is in you.' St. Thomas' reading is apparently taken from Bede.] and hope which is in you." Now the Apostle would not give this advice, if it would imply a diminution in the merit of faith. Therefore reason does not diminish the merit of faith.
I answer that, As stated above (Article 9), the act of faith can be meritorious, in so far as it is subject to the will, not only as to the use, but also as to the assent. Now human reason in support of what we believe, may stand in a twofold relation to the will of the believer. First, as preceding the act of the will; as, for instance, when a man either has not the will, or not a prompt will, to believe, unless he be moved by human reasons: and in this way human reason diminishes the merit of faith. On this sense it has been said above (I-II, 24, 3, ad 1; 77, 6, ad 2) that, in moral virtues, a passion which precedes choice makes the virtuous act less praiseworthy. For just as a man ought to perform acts of moral virtue, on account of the judgment of his reason, and not on account of a passion, so ought he to believe matters of faith, not on account of human reason, but on account of the Divine authority. Secondly, human reasons may be consequent to the will of the believer. For when a man's will is ready to believe, he loves the truth he believes, he thinks out and takes to heart whatever reasons he can find in support thereof; and in this way human reason does not exclude the merit of faith but is a sign of greater merit. Thus again, in moral virtues a consequent passion is the sign of a more prompt will, as stated above (I-II, 24, 3, ad 1). We have an indication of this in the words of the Samaritans to the woman, who is a type of human reason: "We now believe, not for thy saying" (John 4:42).
Reply to Objection 1. Gregory is referring to the case of a man who has no will to believe what is of faith, unless he be induced by reasons. But when a man has the will to believe what is of faith on the authority of God alone, although he may have reasons in demonstration of some of them, e.g. of the existence of God, the merit of his faith is not, for that reason, lost or diminished.
Reply to Objection 2. The reasons which are brought forward in support of the authority of faith, are not demonstrations which can bring intellectual vision to the human intellect, wherefore they do not cease to be unseen.
But they remove obstacles to faith, by showing that what faith proposes is not impossible; wherefore such reasons do not diminish the merit or the measure of faith. On the other hand, though demonstrative reasons in support of the preambles of faith [The Leonine Edition reads: 'in support of matters of faith which are however, preambles to the articles of faith, diminish,' etc.], but not of the articles of faith, diminish the measure of faith, since they make the thing believed to be seen, yet they do not diminish the measure of charity, which makes the will ready to believe them, even if they were unseen; and so the measure of merit is not diminished.
Reply to Objection 3. Whatever is in opposition to faith, whether it consist in a man's thoughts, or in outward persecution, increases the merit of faith, in so far as the will is shown to be more prompt and firm in believing. Hence the martyrs had more merit of faith, through not renouncing faith on account of persecution; and even the wise have greater merit of faith, through not renouncing their faith on account of the reasons brought forward by philosophers or heretics in opposition to faith. On the other hand things that are favorable to faith, do not always diminish the promptness of the will to believe, and therefore they do not always diminish the merit of faith.
Source: Summa Theologica (New Advent)