Distinguishing Faith and Reason. Though faith is not separated from reason, Aquinas does formally distinguish between them. He believed they are related, but the relationship does not coerce a person to believe.

Faith in Relation to Reason. Human reason does not force faith. If it did, then faith would not be a free act. What happens is that "the mind of the one believing settles upon the one side of a question not in virtue of his reason but in virtue of his will. Therefore assent is understood in the definition [of faith] as an act of the mind in so far as the mind is brought to its decision by the will" (ibid., 2a2ae. 2, 1, ad 3).

Faith is not unreasonable. Faith is reason with assent. For "to ponder with assent is, then, distinctive of the believer: this is how his act of belief is set off from all other acts of the mind concerned with the true and the false" (Summa Theologica, 2a2ae.2, 1, reply). Faith, then, is defined as "that habit of mind whereby eternal life begins in us and which brings the mind to assent to things that appear not." Faith differs from science in that the object of faith is unseen. It also differs from doubt, suspicion and opinion in that there is evidence to support faith.

Faith is a free act. Aquinas quotes Augustine with approval that "Faith is a virtue by which things not seen are believed" (ibid., 2a2ae.4, 1, reply). He declares that

Faith is an act of mind and will. Since belief is an act of the intellect under the impetus of the will, it issues from both mind and will, and both are perfectible by action. "If an act of faith is to be completely good, then, habits must necessarily be present in both mind and will" (ibid., 2a2ae. 4, 2, reply). That is, one cannot be saved without a willingness to do something with faith. Saving faith will produce good works.

Meritorious Nature of Faith. Faith is meritorious, not because one has to work for it, but because it involves the will to believe. It "depends on the will according to its very nature (ibid., ad 5). "For in science and opinion [probable arguments] there is no inclination because of the will, but only because of reason" (ibid., 14.3, reply). But "no act can be meritorious unless it is voluntary, as has been said" (ibid., 14.5, reply).

Aquinas believed that Hebrews 11:1 is a good definition of faith, for it describes not merely what faith does but what it is. He saw in it the three essentials:

1. It mentions the will and the object that moves the will as principles on which the       nature of faith is based.

2. In it we can distinguish faith from those things which appear not, as opposed to       science and understanding.

3. The whole definition reduces to the essential phrase, "the substance of things hoped       for." (ibid., 14.2)

The formal difference between faith and reason is that one cannot both know and believe the same thing at the same time. For "Whatever things we know with scientific knowledge properly so called we know by reducing them to first principles which are naturally present to the understanding."

Faith and Knowledge about the Same Object. Scientific knowledge culminates in sight of the thing believed, so there is no room for faith. One cannot have faith and scientific knowledge about the same thing (ibid., 14.9, reply). The object of true faith is above senses and understanding. "Consequently, the object of faith is that which is absent from our understanding." As Augustine said, "we believe that which is absent, but we see that which is present" (ibid., 14.9, reply).

This does not mean, of course, that everyone will necessarily believe what I can see without faith (Summa Theologica, 2a2ae. 1, 5). It does mean that the same person cannot have both faith and proof of the same object. One who sees it, does not believe it by faith on the testimony of others. One who believes it on the testimony of another does not see (know) it personally.

Probable Knowledge and Faith. Likewise, one cannot have "opinion" (probable knowledge) and "science" (certain knowledge) about the same object. As Aquinas notes, "opinion includes a fear that the other part [of the contradiction] is true, and scientific knowledge excludes such fear. However, this fear that the opposite may be true does not apply to matters of faith. For faith brings with it a greater certitude than what can be known by reason" (On Truth, 14.9, ad 6).

Creedal Knowledge and Faith. If the existence of God can be proved by reason, and if what is known by reason cannot also be a matter of faith, then why is belief in God proposed in the Creed? Aquinas responds that not all are capable of demonstrating God’s existence. "We do not say that the proposition. God is one, in so far as it is proved by demonstration, is an article of faith, but something presupposed before the articles. For the knowledge of faith presupposes natural knowledge, just as grace presupposes nature" (ibid., 14.9, ad 8).

Perfected, by Love, Produced by Grace. Reason can go only so far. Faith goes beyond reason and completes it. "Faith does not destroy reason, but goes beyond it and perfects it" (ibid., 14.10, reply, ad 7). "Love is the perfection of faith. Since charity is a perfection of the will, faith is formed by charity" (ibid., ad 1). "It is called form in so far as faith acquires some perfection from charity" (ibid., ad 7). But "the act of faith which precedes charity is an imperfect act awaiting completion from charity" (ibid., 14.A5, reply). So love perfects faith. Since believing depends on the understanding and the will, "such an act cannot be perfect unless the will is made perfect by charity and the understanding by faith. Thus formless faith cannot be a virtue" (ibid., ad 1).

However, "that which faith receives from charity is accidental to faith in its natural constitution, but essential to it with reference to its morality" (ibid., 14.6, reply).

Not only is love necessary to perfect faith, but grace is necessary to produce it. "Now, grace is the first [that is, remote] perfection of the virtues, but charity is their proximate perfection" (ibid., 14.A5, ad 6).

The Limitations of Reason. Aquinas did not believe that human reason was without limitations. In fact he offered many arguments as to why reason is insufficient and revelation is needed.

Five Reasons for Revelation. Following Jewish philosopher Moses Maimonides, Aquinas set forth five reasons why we must first believe what we may later be able to provide good evidence for (Maimonides, 1.34):

1. The object of spiritual understanding is deep and subtle, far removed from sense perception.

2. Human understanding is weak as it fights through these issues.

3. A number of things are needed for conclusive spiritual proof. It takes time to discern them.

4. Some people are disinclined to rigorous philosophical investigation.

5. It is necessary to engage in other occupations besides philosophy and science to provide the necessities of life (On Truth, 14.10, reply).

Aquinas said it is clear that, "if it were necessary to use a strict demonstration as the only way to reach a knowledge of the things which we must know about God, very few could ever construct such a demonstration and even these could do it only after a long time." Elsewhere, Aquinas lists only three basic reasons divine revelation is needed.

1. Few possess the knowledge of God, some do not have the disposition for philosophical study, and others do not have the time or are indolent.

2. Time is required to find the truth. This truth is very profound, and there are many  things that must be presupposed. During youth the soul is distracted by "the various  movements of the passions."

3. It is difficult to sort out what is false in the intellect. Our judgment is weak in sorting  true from false concepts. Even in demonstrated propositions there is a mingling of  false.

"That is why it was necessary that the unshakable certitude and pure truth concerning divine things should be presented to men by way of faith" (Gentiles, 1.4, 2-5).

The Noetic Effects of Sin. Clearly, the mind falls far short when it comes to the things of God. As examples of weakness Aquinas looked at the philosophers and their errors and contradictions. "To the end, therefore, that a knowledge of God, undoubted and secure, might be present among men, it was necessary that divine things be taught by way of faith, spoken as it were by the Word of God who cannot lie" (ibid., 2a2ae. 2, 4). For "the searching of natural reason does not fill mankind’s need to know even those divine realities which reason could prove" (ibid., 2a2ae.2, 4, reply).

As a result of the noetic effects of sin, grace is needed. Aquinas concluded that "If for something to be in our power means that we can do it without the help of grace, then we are bound to many things that are not within our power without healing grace—for example to love God or neighbor." The same is true of belief. But with the help of grace we do have this power (ibid., 2a2ae.2, 6, ad 1).

However, Aquinas did not believe that sin destroyed human rational ability. "Sin cannot destroy man’s rationality altogether, for then he would no longer be capable of sin" (ibid., 1a2ae.85, 2).

Things above Reason. Not only is faith necessary because of human depravity, but also because some things simply go beyond the power of reason. That does not mean they are contrary to reason, but that they are not fully comprehensible. "Faith, however, is said to surpass reason, not because there is no act of reason in faith, but because reasoning about faith cannot lead to the sight of those things which are matters of faith" (ibid., 14.A2, ad 9). If one could base faith fully on reason, faith would not be a free act; it would be consent caused by the mind.

At two levels a matter of faith may be "above reason." At its highest level it can be above reason absolutely—if it exceeds the intellectual capacity of the human mind (e.g., the Trinity). It is impossible to have scientific knowledge of this. Believers assent to it only on the testimony of God." Or, it may not absolutely exceed the intellect capacity of all, but is exceedingly difficult to comprehend, and is above the intellectual capacity of some (for example, that God exists without body). "These we may have scientific proofs of and, if not, we may believe them" (On Truth, 14.9, reply).

We must have faith when the light of grace is stronger than the light of nature. For "although the divinely infused light is more powerful than natural light, in our present state we do not share it perfectly, but imperfectly." Therefore, "because of this defective participation, through that infused light itself we are not brought to the vision of those things for the knowledge of which it was given us. But we will have it in heaven when we share that light perfectly and in the light of God we will see light" (Gentiles, 14.8, ad 2).

Faith, then, surpasses reason. For "some truths about God exceed all the ability of the human reason. Such is the truth that God is triune" (ibid., 1.3). The ineffable essence of God cannot be known by human reason. The reason for this is that the mind depends on the senses. "Now, sensible things cannot lead the human intellect to the point of seeing in them the nature of the divine substance; for sensible things are effects that fall short of the power of their cause" (ibid., 1.3,3).

Just because we have no reasons for things that go beyond reason does not mean they are not rational. Every belief that is not self-evident can be defended as necessary. We may not know the argument, but it exists. It at least is known to God "and to the blessed who have vision and not faith about these things" (De Trinitate, 1.1.4; On Truth, 14.9, ad 1). While human reason cannot attain to the things of faith, it is the preface to them. While "philosophical truths cannot be opposed to truths of faith, they fall short indeed, yet they also admit common analogies; and some moreover are foreshadowing, for nature is the preface of grace" (De Trinitate, 2.3).

"Although the truth of the Christian faith which we have discussed surpasses the capacity of the reason, nevertheless that truth that the human reason is naturally endowed to know cannot be opposed to the truth of the Christian faith" (Gentiles, 1.7, [1]).

Summary. Aquinas’s view of the relation of faith and reason blends positive elements of presuppositionalism and evidentialism, of rationalism and fideism. Aquinas stresses the need for reason before, during, and after beliefs are acquired. Even the mysteries of faith are not irrational.

On the other hand, Aquinas does not believe that reason alone can bring anyone to faith. Salvation is accomplished only by the grace of God. Faith can never be based on reason. At best it can only be supported by reason. Thus, reason and evidence never coerce faith. There is always room for unbelievers not to believe in God, even though a believer can construct a valid proof that God exists. Reason can be used to demonstrate that God exists, but it can never in itself persuade someone to believe in God. Only God can do this, working in and through their free choice.

These distinctions of Aquinas are eminently relevant to the discussion between rationalists and fideists or between evidentialists and presuppositionalists. With regard to belief that God exists, Aquinas sides with the rationalists and evidentialists. But with respect to belief in God, he agrees with fideists and presuppositionalists.

(from Baker Encyclopedia of Christian Apologetics, Baker Book House, 1999)