Objection 1. It would seem unnecessary for salvation to believe anything above the natural reason. For the salvation and perfection of a thing seem to be sufficiently insured by its natural endowments. Now matters of faith, surpass man's natural reason, since they are things unseen as stated above (Question 1, Article 4). Therefore to believe seems unnecessary for salvation.
Objection 2. Further, it is dangerous for man to assent to matters, wherein he cannot judge whether that which is proposed to him be true or false, according to Job 12:11: "Doth not the ear discern words?" Now a man cannot form a judgment of this kind in matters of faith, since he cannot trace them back to first principles, by which all our judgments are guided. Therefore it is dangerous to believe in such matters. Therefore to believe is not necessary for salvation.
Objection 3. Further, man's salvation rests on God, according to Psalm 36:39: "But the salvation of the just is from the Lord." Now "the invisible things" of God "are clearly seen, being understood by the things that are made; His eternal power also and Divinity," according to Romans 1:20: and those things which are clearly seen by the understanding are not an object of belief. Therefore it is not necessary for man's salvation, that he should believe certain things.
On the contrary, It is written (Hebrews 11:6): "Without faith it is impossible to please God."
I answer that, Wherever one nature is subordinate to another, we find that two things concur towards the perfection of the lower nature, one of which is in respect of that nature's proper movement, while the other is in respect of the movement of the higher nature. Thus water by its proper movement moves towards the centre (of the earth), while according to the movement of the moon, it moves round the centre by ebb and flow. On like manner the planets have their proper movements from west to east, while in accordance with the movement of the first heaven, they have a movement from east to west. Now the created rational nature alone is immediately subordinate to God, since other creatures do not attain to the universal, but only to something particular, while they partake of the Divine goodness either in "being" only, as inanimate things, or also in "living," and in "knowing singulars," as plants and animals; whereas the rational nature, in as much as it apprehends the universal notion of good and being, is immediately related to the universal principle of being.
Consequently the perfection of the rational creature consists not only in what belongs to it in respect of its nature, but also in that which it acquires through a supernatural participation of Divine goodness. Hence it was said above (I-II, 3, 8) that man's ultimate happiness consists in a supernatural vision of God: to which vision man cannot attain unless he be taught by God, according to John 6:45: "Every one that hath heard of the Father and hath learned cometh to Me." Now man acquires a share of this learning, not indeed all at once, but by little and little, according to the mode of his nature: and every one who learns thus must needs believe, in order that he may acquire science in a perfect degree; thus also the Philosopher remarks (De Soph. Elench. i, 2) that "it behooves a learner to believe."
Hence in order that a man arrive at the perfect vision of heavenly happiness, he must first of all believe God, as a disciple believes the master who is teaching him.
Reply to Objection 1. Since man's nature is dependent on a higher nature, natural knowledge does not suffice for its perfection, and some supernatural knowledge is necessary, as stated above.
Reply to Objection 2. Just as man assents to first principles, by the natural light of his intellect, so does a virtuous man, by the habit of virtue, judge aright of things concerning that virtue; and in this way, by the light of faith which God bestows on him, a man assents to matters of faith and not to those which are against faith. Consequently "there is no" danger or "condemnation to them that are in Christ Jesus," and whom He has enlightened by faith.
Reply to Objection 3. In many respects faith perceives the invisible things of God in a higher way than natural reason does in proceeding to God from His creatures. Hence it is written (Sirach 3:25): "Many things are shown to thee above the understandings of man."
Source: Summa Theologica (New Advent)