Faith and Reason/Part 1

By: Dr. Norman Geisler; ©2003
The relation of faith to reason is of utmost importance for the thinking believer. Dr. Geisler helps us sort through the arguments involved.


The relation of faith to reason is of utmost importance for the thinking believer. The problem of how to combine these aspects of personhood has existed from the earliest apologists. Justin Martyr, Clement of Alexandria, and Tertullian all struggled. Augustine made the first serious attempt to relate the two, but the most comprehensive treatment came at the end of the medieval period when Christian intellectualism flowered in the work of Thomas Aquinas.

Relation of Faith to Reason

Aquinas held that Faith and Reason intertwine. Faith uses reason, and reason cannot succeed in finding truth without faith.

Reason Cannot Produce Faith. Reason accompanies, but does not cause, faith. Faith is consent without inquiry in that faith’s assent is not caused by investigation. Rather, it is produced by God. Commenting on Ephesians 2:8-9, 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” (Aquinas, Ephesians, 96; unless noted, all citations in this article are from works by Thomas 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” (On Truth, 14.A1.2). Such discursive thought, or reasoning from premises to conclusions, is not the cause of the assent of faith, but it can and should accompany it (ibid., 14.A1.6). Faith and Reason are parallel. One does not cause the other because “faith involves will (freedom) and reason doesn’t coerce the will” (ibid.). A person is free to dissent, even though there may be convincing reasons to believe.

As a matter of tactical approach in apologetics, if the authority of Scripture is accepted (faith), appeal can be made to it (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 and the pagans accept neither the one nor the other…. We must, therefore, have recourse to the natural reason, to which all men are forced to give their assent” (Summa Theologica, 1a.2.2).

However, some Christian truths are attainable by human reason, for example, that God exists and is one. “Such truths about God have been proved demonstratively by the phi­losophers, guided by the light of the natural reason” (ibid., 1a.3.2)

Three Uses of Reason

Reason or philosophy can be used in three ways, Aquinas says:

  1. It demonstrates the “preambles of faith” (that God exists, that we are his creatures…)
  2. It analyzes teachings of philosophers in order to reveal corresponding concepts in Chris­tian faith. Aquinas gives the example of Augustine’s On the Trinity, which draws on phi­losophy to help explain the Trinity.
  3. It opposes attacks against faith from logic (Gentiles, 1.9).

Reason can be used to prove natural theology, which studies the existence and nature of one God. It can be used to illustrate supernatural theological concepts, such as the Trinity and the Incarnation. And it can be used to refute false theologies (De Trinitate, 2.3). The apologist directs the person to accept two kinds of truth about divine things and to destroy what is contrary to truth. The person is directed to the truths of natural theology by the investigation of the reason and to the truths of supernatural theology by faith.

So to make the first kind of divine truth known, we must proceed through demonstrative arguments. However,

…since such arguments are not available for the second kind of divine truth, our intention should not be to convince our adversary by arguments: It should be to answer his arguments against the truth; for, as we have shown, the natural reason cannot be contrary to the truth of faith. 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. [Gentiles, 1.9]

God’s existence is self-evident absolutely (in itself) but not relatively (to us) (ibid., 1.10- 11). Hence, in the final analysis, one must receive by faith those things that can be known by reason, as well as those things that lie above reason. Intellectual assent that lacks faith cannot have certitude, for human reason is notoriously suspect when it comes to spiritual matters. Consequently, “it was necessary for divine truth to be delivered by way of faith, being told to them as it were, by God Himself Who cannot lie” (Summa Theologica, 2a2ae.1, 5.4).

Divine Authority

Aquinas did not believe that reason provides the basis for believing in God. It can prove that God exists, but it cannot convince an unbeliever to believe in God.

Reason Prior to Faith. We may believe (assent without reservation) in something that is neither self-evident nor deduced from it by a movement of the will. However, this does not mean that reason plays no prior role to belief. We judge a revelation to be worthy of belief “on the basis of evident signs or something of the sort” (ibid., 2a2ae.1, 4. ad 2).

Reason inquires about what is to be believed before it believes in it. “Faith does not involve a search by natural reason to prove what is believed. But it does involve a form of inquiry unto things by which a person is led to belief, e.g. whether they are spoken by God and confirmed by miracles” (ibid., 2a2ae.2, 1, reply). Demons are not willingly convinced by the evidence that God exists but are intellectually forced by confirming signs to the fact that what the faithful believe is true. Yet they cannot truly be said to believe (On Truth, 14.9. ad 4).

The Testimony of the Spirit. 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” (Summa Theologica, 2a2ae.6.1). The Holy Spirit uses two causes to stimulate voluntary faith. The persuasion may be from with­out, for example, a miracle that is witnessed. Or persuasion may be from within. The first cause is never enough for one inwardly to assent to the things of faith. The assent of faith is caused by God as he moves the believer inwardly through grace. Belief is a matter of the will, but the will needs to be prepared by God “to be lifted up to what surpasses nature” (ibid., 2a2ae.2, 9. ad 3).

Reason in Support of Faith

Commenting on the use of reason in 1 Peter 3:15, Aquinas argued that “human reasoning in support of what we believe may stand in a two­fold relation to the will of the believer.” First, the unbeliever may not have the will to believe unless moved by human reason. Second, the person with a will ready to believe loves the truth, thinks it out, and takes to heart its evidence. The first, unbelieving will may come to a faith of sorts, but there will be no merit in it, because belief does not extend far beyond sight. The second person also studies the human reasoning, but it is a meritorious work of faith (ibid., 2a2ae.2, 10).

Positive Evidence. 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’” (2 Peter 1:16). 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.” The kind of positive evidence that Aquinas used included such things as raising the dead, miracles, and the conversion of the pagan world to Christianity (On Truth, 14.A1).

Negative Evidence. The negative evidence encompasses arguments against false religions, including things like their fleshly appeal to carnal pleasures, their teachings that contradict their promises, their many fables and falsities, the lack of miracles to witness to divine inspiration of their holy book (like the Qur’an), use of warfare (arms) to spread their message, the fact that wise men did not believe Muhammad, only ignorant, desert wander­ers, the fact that there were no prophets to witness to him, and Muslim perversions of Old and New Testament stories (Gentiles, 1.6).

Faith and Fallible Testimony. How can we be sure when the support of our faith rests on many intermediary (fallible) testimonies? Aquinas responds that the intermediaries are above suspicion if they were confirmed by miracles (for example, Mark 16:20). “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” (On Truth, 14.10, ad 11). The Bible alone is the final and infallible authority for our faith.

Faith and Demonstrative Arguments. Aquinas distinguished between two kinds of ratio­nal arguments: demonstrative and persuasive. “Demonstrative, cogent, and intellectually 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” (De Trinitate, 2.1, ad 5).

Read Part 2

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