Objection 1. It would seem unnecessary to believe those things which can be proved by natural reason. For nothing is superfluous in God's works, much less even than in the works of nature. Now it is superfluous to employ other means, where one already suffices. Therefore it would be superfluous to receive by faith, things that can be known by natural reason.
Objection 2. Further, those things must be believed, which are the object of faith. Now science and faith are not about the same object, as stated above (1, 4,5). Since therefore all things that can be known by natural reason are an object of science, it seems that there is no need to believe what can be proved by natural reason.
Objection 3. Further, all things knowable scientifically [Science is certain knowledge of a demonstrated conclusion through its demonstration] would seem to come under one head: so that if some of them are proposed to man as objects of faith, in like manner the others should also be believed. But this is not true. Therefore it is not necessary to believe those things which can be proved by natural reason.
On the contrary, It is necessary to believe that God is one and incorporeal: which things philosophers prove by natural reason.
I answer that, It is necessary for man to accept by faith not only things which are above reason, but also those which can be known by reason: and this for three motives. First, in order that man may arrive more quickly at the knowledge of Divine truth. Because the science to whose province it belongs to prove the existence of God, is the last of all to offer itself to human research, since it presupposes many other sciences: so that it would not by until late in life that man would arrive at the knowledge of God. The second reason is, in order that the knowledge of God may be more general. For many are unable to make progress in the study of science, either through dullness of mind, or through having a number of occupations, and temporal needs, or even through laziness in learning, all of whom would be altogether deprived of the knowledge of God, unless Divine things were brought to their knowledge under the guise of faith. The third reason is for the sake of certitude. For human reason is very deficient in things concerning God. A sign of this is that philosophers in their researches, by natural investigation, into human affairs, have fallen into many errors, and have disagreed among themselves. And consequently, in order that men might have knowledge of God, free of doubt and uncertainty, it was necessary for Divine matters to be delivered to them by way of faith, being told to them, as it were, by God Himself Who cannot lie.
Reply to Objection 1. The researches of natural reason do not suffice mankind for the knowledge of Divine matters, even of those that can be proved by reason: and so it is not superfluous if these others be believed.
Reply to Objection 2. Science and faith cannot be in the same subject and about the same object: but what is an object of science for one, can be an object of faith for another, as stated above (Question 1, Article 5).
Reply to Objection 3. Although all things that can be known by science are of one common scientific aspect, they do not all alike lead man to beatitude: hence they are not all equally proposed to our belief.
Source: Summa Theologica (New Advent)