by Norman L Geisler

Thomas Aquinas was born near Aquino, Italy probably around 1224-5. He attended the University of Naples in 1239 where he encountered the writings of Aristotle which was to have a great impact on his thinking. Aquinas joined the Dominican Order of Preachers around 1243—in spite of the violent opposition of his family.1  Later he studied and then taught at the University of Paris. Aquinas died on March 4, 1274, at Fossanova on his way to the Council of Lyons.

Aquinas wrote some ninety works, many of which are multivolume sets. His works represent the whole gamut of theological, philosophical, biblical, and ethical topics.  The most famous and mature work is the Summa Theologiae, one of the most massive and systematic theologies ever produced.  Other well-known works include the Summa Contra Gentiles and On Being and Essence.  Because of his specialized work on angels, Thomas Aquinas has been called by the church, “The Angelic Doctor.”

The apologetics of Aquinas is widely misunderstood among evangelicals.  This is due largely to two influential evangelical thinkers who did not understand Aquinas’ views—Francis Schaffer and Cornelius Van Till.  Notable exceptions to this kind of misunderstanding by evangelicals are found in John Gerstner, R. C. Sproul, Arvin Vos, Win Corduan, Stuart Hackett, and David Beck.  Reasons for misunderstanding of Aquinas include: (1) the failure to read him, rather than secondary sources on him; (2) a prior bias against his views, or (3) the widespread attitude expressed in the question: “Can anything good come out of Rome?”  As we shall see, the answer is “Yes—a lot!”  Even non-Catholics can find in Aquinas a helpful way to preserve the evangelical Faith. This will become evident in the following discussion of the core apologetic issue—the relation of faith and reason.


Reason Cannot Produce Faith1

For Thomas, reason accompanies faith, but it does not cause faith. “Faith is called a consent without inquiry in so far as the consent of faith, or assent, is not caused by an investigation of the understanding.”  Faith is produced by God.  Commenting on Ephesians 2:8-9 (in his Commentary on Paul’s Epistles), Aquinas contended that “free will is inadequate for the act of faith since the contents of faith are above reason…. That a man should believe, therefore, cannot occur from himself unless God gives it….”2 So, for Aquinas, faith is a gift of God, and no one can believe without it.

Nonetheless, “this does not prevent the understanding of one who believes from having some discursive thought of comparison about those things which he believes.”3  Nonetheless, such reasoning “can accompany the assent of faith.”4  The reason they are parallel but one does not cause the other is that “faith involves will (freedom) and reason doesn’t coerce the will.”5  That is, a person is free to dissent even though there may be convincing reasons to believe.

So, in proclaiming Christian truth to others who do not accept the authority of Scripture, appeal can be made to reason. “Thus, against the Jews we are able to argue by means of the Old Testament, while against heretics we are able to argue by means of the New Testament.  But Mohammedans [Muslims] and the pagans accept neither the one nor the other.”  Aquinas concludes: “we must, therefore, have recourse to the natural reason, to which all men are forced to give their assent.”6 Thus, some truths of the Christian Faith are attainable by human reason. “Such are that God exists, that He is one, and the like.  In fact, such truths about God have been proved demonstratively by the philosophers, guided by the light of the natural reason.”7  Such, Aquinas believes, are his famous “Five Ways” to prove the existence of God.8

The Three Uses of Reason in the Christian Faith9

Aquinas believed that “in Sacred Teaching we can use philosophy in a threefold way.”  First, “we can use it to demonstrate the preambles of faith. . . for example, that God exists, that God is one, or similar propositions concerning God or creatures that faith proposes as having been proved in philosophy.” Second, “we can use philosophy to make known through certain likenesses [analogies] what belongs to faith, as Augustine in his book On the Trinity uses many likenesses drawn from the teachings of the philosophers to explain the Trinity.”  Third, “we can use philosophy to oppose what is said against faith, either by showing that these things are false or by showing that they are not necessary.”10  In brief, human reason can be used to prove natural theology (the existence and nature of one God) which is based on general revelation (cf. Rom. 1:19-20).  Also, it can be used to illustrate supernatural theology (the Trinity and Incarnation) which are known only from special revelation in the Bible.  Also, human reason can be used to refute false theologies (heresies).11


Faith is More Certain than Reason

Aquinas says that “the wise man ought to be directed toward the twofold truth of divine things, and toward the destruction of the errors that are contrary to this truth.  One kind of divine truth the investigation of the reason is competent to reach, whereas the other surpasses every effort of the reason.”  However, the sole way to overcome an adversary of divine truth is from the authority of Scripture—an authority divinely confirmed by miracles.  For that which is above the human reason we believe only because God has revealed it.  Nevertheless, there are certainly likely [probable] arguments that should be brought forth in order to make divine truth known.”12

God’s existence is self-evident absolutely (in itself) but not relatively (to us).13   Hence, it “is necessary for man to receive by faith not only things which are above reason, but also those which can be known by reason.”  Furthermore, without faith we would lack certitude. “For human reason is very deficient in things concerning God. . . .”  Consequently, “in order that men might have knowledge of God, free of doubt and uncertainty, it was necessary for divine truth to be delivered by them by way of faith, being told to them as it were, by God Himself Who cannot lie.”14


The Basis of Faith is Divine Authority

Reason in itself cannot provide the basis for believing in God.  At best it can only prove that God exists, but it cannot convince an unbeliever to believe in God.  For “they do not truly believe in God.”15   We may believe (assent without reservation) in something which is neither self-evident nor deduced from it (where the intellect is moved) by a movement of the will.

However, this does not mean that reason plays no role prior to belief.  We must judge a revelation to be worthy before we believe it.  For “he would not believe unless he saw that they are worthy of belief on the basis of evident signs or something of the sort.”16  Indeed, reason enquires about what is to be believed before it believes in it.  In Thomas’s words, “Faith does not involve a search by natural reason to prove what is believed.  But it does involve a form of enquiry unto things by which a person is led to belief, e.g.,  whether they are spoken by God and confirmed by miracles.”17  Demons, for example, are convinced by the evidence that God exists but it “is not their wills which bring demons to assent to what they are said to believe.  Rather, they are forced by the evidence of signs which convince them that what the faithful believe is true.”  But this is not saving faith, for  “belief is predicated equivocally of men who believe and of demons.”18


The Testimony of the Holy Spirit

According to Aquinas, in order to believe in God, one must have the inner testimony of the Holy Spirit.  For “one who believes does have a sufficient motive for believing, namely the authority of God’s teaching, confirmed by miracles, and—what is greater—the inner inspiration [instinctus] of God inviting him to believe.”19  As to voluntary assent in matters of faith, we can look to two types of causes:  “One is a cause that persuades from without, for example, a miracle witnessed or a human appeal urging belief.”  Second, “No such cause is enough, however; one man believes and another does not, when both have seen the same miracle, heard the same preaching.  Another kind of cause must therefore be present, an inner cause; one that influences a person inwardly to assent to the things of faith.”  Hence, “The assent of faith,  which is its principal act, therefore, has as its cause God, moving us inwardly through grace.”  This “belief is, of course, a matter of the believer’s will, but a person’s will needs to be prepared by God through his grace in order to be lifted up to what surpasses nature . . .”20


Human Reason Can be Used to Support Faith21

Commenting on the use of  “reason” in 1 Peter 3:15, Thomas argued that “human reasoning in support of what we believe may stand in a two-fold 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 reason; and in this way human reasoning diminishes the merit [value] of faith . . . he ought to believe matters of faith, not because of human reasoning, but because of the divine authority.”  Secondly, “human reasons may be consequent to the will of the believer.  For when a man has a will ready to believe, he loves the truth he believes, he thinks out and takes to heart whatever reasons he can find in support thereof.”22

So, faith is supported by, though not based on, probable evidence. “Those who place their faith in this truth, however, ‘for which the human reason offers no experimental evidence,’ do not believe foolishly, as though ‘following artificial fables.’”23  Rather, “It reveals its own presence, as well as the truth of its teaching and inspiration, by fitting arguments; and in order to confirm those truths that exceed natural knowledge, it gives visible manifestations to works that surpass the ability of all nature.”24  The kind of positive evidence that Aquinas used in support faith included things like raising the dead, the conversion of the world to Christianity, and miracles.

The negative evidence encompasses arguments against false religions, including things like Muslim’s fleshly appeal to carnal pleasures (for their Jihadist after death), their teachings that contradict their promises, their many fables and falsities, the lack of miracles to witness to divine inspiration of their holy book (the Qur’an), use of warfare (arms) to spread their message, the fact that wise men did not believe Mohammed, only ignorant, desert wanderers did, the fact that there were no prophets to witness to him, and that Muslims perverted Old Testament and New Testament stories.25


The Relation of Faith and Human Witnesses

 How we can be sure when the support of our faith rests on many intermediary human testimonies?  Aquinas responded: ‘All the intermediaries through which faith comes to us are above suspicion.  We believe the prophets and apostles because the Lord has been their witness by performing miracles, as Mark says [16:20]….”  Furthermore, “we believe the successors of the apostles and prophets only in so far as they tell us those things which the apostles and prophets have left in their writings.”26  The Bible alone is the final and infallible authority for our Faith.

Aquinas distinguished between two kinds of rational arguments: demonstrative and persuasive. “Demonstrative, cogent, and intellectual convincing argument cannot lay hold of the truths of faith, though it may neutralize destructive criticism that would render faith untenable.”  On the other hand, “persuasive reasoning drawn from probabilities . . . does not weaken the merit of faith, for it implies no attempt to convert faith into sight by resolving what is believed into evident first principles.”27


Faith In Relation to Reason

While Aquinas did not actually separate faith and reason, nonetheless,  he did formally distinguish them.  He believed they were related, but the relationship was not coercive.28   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.”29  This does not mean that faith is 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.”30


God as an Object of Reason and Faith

Aquinas held that one cannot both know and believe the same thing at the same time.  For “Whatever things we know with scientific [i.e., philosophic] knowledge properly so called we know by reducing them to first principles which are naturally present to the understanding.”  Thus, “all scientific knowledge terminates in the sight of a thing which is present [whereas faith is always in something absent].  Hence, it is impossible to have faith and scientific knowledge about the same thing.” 31  The object of faith is above senses and understanding.

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’ response is that not all men 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.”32


The Limitations of Reason

Also, contrary to a popular opinion, Aquinas did not believe that human reason was without limitations.  In fact he offered many arguments why reason is insufficient and revelation is needed.  Five reasons for needing revelaton are given.33  Following the famous Jewish philosopher, Moses Maimonides,34 Aquinas said, “The first reason is the depth and subtlety of these objects of knowledge which are farthest removed from the senses . . . . The second reason is the weakness of human understanding when it begins to operate.  The third is the number of things needed for a conclusive proof of these.  A man can learn them only after a long time.  The fourth reason is the disinclination for scientific investigation which some men have because they lack the proper temperament. The fifth is the need of engaging in other occupations to provide the necessities of life.35


The Noetic Effects of Sin36

There is another widely held but mistaken view that Aquinas believed the mind was only finite but not fallen.  This is contrary to his clear statement that “the mind of man falls far short when it comes to the things of God.  Look at the  philosophers; even in searching into questions about man they have erred in many points and held contradictory views.  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.”37  For “the searching of natural reason does not fill mankind’s need to know even those divine realities which reason could prove. Belief in them is not, therefore, superfluous.38  Aquinas asserted emphatically that: “human reason is very deficient in things concerning God.  A sign of this is that philosophers, in their inquiry into human affairs by natural investigation, 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 truths to be delivered to them by way of faith, being told to them as it were, by God Himself Who cannot lie.39  Of course, God’s image in fallen man is effaced but not erased.

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.”  Further, “the same is true of believing in the articles of faith.  But with the help of grace we do have this power.  As Augustine says, to whomever this help is given by God it is given in mercy; to whomever it is denied, it is denied in justice, namely because of previous sin, even if only original sin.”40  However, Aquinas did not believe that sin completely destroyed man’s rational ability.  Rather, he insisted that “sin cannot destroy man’s rationality altogether, for then he would no longer be capable of sin.”41

Some Things are Above Reason

Not only is faith necessary because of human depravity, but also because in some things God is simply beyond the power of reason.  By “beyond reason” he does not mean what is contrary to reason but simply what cannot be comprehended by reason.  As Aquinas puts it, they are the “things which are beyond the comprehension of reason, and so to this extent seem to contradict reason, although they do not really oppose it.42  “Above reason” can have two meanings.  It can mean above reason absolutely—“if it exceeds the intellectual capacity of all men in this life (e.g., that there is a Trinity).  It is impossible to have scientific knowledge of this.  Believers assent to it only on the testimony of God.”  Or, it may mean above reason not absolutely—“if it doesn’t exceed the intellect capacity of all men but only some men (e.g., that God exists, has no body, etc.).”43  These we may have scientific proofs of and, if not, we may believe them but not both (for one and the same person).  This does not mean that faith opposes reason. For “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.”44


Aquinas’ apologetic is expressed in his view of the relation of faith and reason which is a unique blend of the positive elements of both presuppositionalism and classical theism.  He stresses the need for reason both before, during, and after believing.  Even the mysteries of faith are not irrational.  But true faith in God comes only by the grace of God.  Indeed, he believes that faith can never be based on reason.  At best it can only be supported by reason.  Thus, reason and evidence are never coercive of faith.  There is always room for unbelievers not to believe in God, even though a believer can construct a valid proof that God exists.  For 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 along with the evidence in and through their free choice.

These distinctions of Aquinas are eminently relevant to the discussion today between rationalists and fideists or between classical theists and  presuppositionalists.  With regard to belief that God exists, Aquinas sides with the former.  But with respect to belief in God, he agrees with the latter.  This unique synthesis of faith and reason provides further reasons to believe that old Aquinas should not be forgotten nor never brought to mind.

Norman L. Geisler is the President of Southern Evangelical Seminary and the author of Thomas Aquinas: An Evangelical Appraisal (Baker, 1991).


1 His family actually imprisoned him for over a year in the hope of persuading him to give up his religious ambitions.

2  For a more thorough discussion of thought and apologetics of Thomas Aquinas, see our book: Thomas Aquinas: An Evangelical Appraisal (Eugene, OR: Wipf and Stock reprint, 2008).

3 Aquinas, Commentary on Saint Paul’s Epistle to the Ephesians (Albany, New York: Magi Books, Inc., 1966), 96.

4 On Truth, XIV, A1, ad. 2)

5 Ibid., XIV, A1, ad 6.

6 Ibid., XIV, A1. ad 6.

7 Summa Theologiae, 1a. 2, 2.

8 Ibid., 1a. 3, 2)

9 See Ibid., 1.2.3.

10 Summa Contra Gentiles, I, 9.

11 Aquinas, Commentary on Saint Paul’s Epistle to the Ephesians, 96.

12 In Boethius, De Trinitate, II, 3.

13 Summa Contra Gentiles.

14 Ibid., I, 10, 11.

15 Summa Theologiae, 2a2ae. 1, 5, ad 4.

16 Ibid., 2a2ae. 2, 2, ad 3.

17 Ibid., 2a2ae. 1, 4, ad 2.

18 Ibid., 2a2ae. 2, 1 reply.

19 On Truth, XIV, 9, ad 4.

20  See also Summa Theologiae, 2a2ae. 6, 1.

21 Ibid., 2a2ae. 2, 9, ad 3.

22 On Truth, XIV, A1, ad 2.

23 Summa Theologiae, 2a2ae. 2, 10 reply.

24 II Peter 1:16.

25 Aquinas, Summa contra Gentiles I, 6.

26 Ibid., I, 6.

27 On Truth, XIV, 10, ad 11.

28 De Trinitate, II, 1, ad 5.

29 Summa Theologiae, 2a2ae. 2, 9.

30 Ibid., 2a2ae. 2, 1, ad 3.

31 Ibid., 2a2ae. 2, 1, reply.

32 On Truth, XIV, 9, reply; Summa Theologiae, 2a2ae. 1, 5.

33 Ibid., XIV, 9, ad 8.

34 Ibid., XIV, 10.

35 Moses Miamonides, A Guide for the Perplexed, I, 34.

36 On Truth, XIV, 10, reply.

37 Summa Theologiae, 2a2ae. 2, 6, ad 1.

38  Ibid., 2a2ae. 2, 4.

39 Ibid., 2a2ae. 2, 4, reply.

40 Ibid., 2a2ae. 2, 4.

41 Ibid., 2a2ae. 2, 6, ad 1.

42 Ibid., 1a2ae. 85, 2.

43 On Truth, XIV, 10, ad 7.

44 Ibid., XIV, 9, reply.

45 Summa Contra Gentiles, I, 7, [1].