Does St. Thomas treat atheism explicitly? No, there is no explicit treatment of atheism in St. Thomas works, not even the use of the word.³⁶ A possible reason for this is that religious belief became civil law both in Christian and Moslem countries.

Did St. Thomas treat atheism implicitly? Yes, St. Thomas attacked the three great arguments that atheists used to refute the existence of God.³⁷ Those arguments are: the argument from science, which made the hypothesis of God useless; the argument from evil, which compromises the reality of God as creator and provident; and the argument from liberty, which appears to be incompatible with an omniscent and omnipotent God. Evolutionary Atheism asserts science eliminates God,³⁸ but St. Thomas answers: "Certainly nature has its operations, which science can investigate and discover, but because they move toward a determined goal under the direction of a superior agent, it is also necessary they are attributed to God as their prime cause" (Aquinas Summa Theologiae 1. 2. 3. ad 1).³⁹ Evolutionary Atheism asserts that evil in the world eliminates God,⁴⁰ but St. Thomas answers: "As St. Augustine says: ‘God being the summit of goodness would never permit His works to be evil if there was not such power and such good from which He knows how to draw good even from evil.’ Thus it pertains to the infinite goodness of God to allow evil in order to turn it to good" (Aquinas Summa Theologiae 1. 2. 3. ad 2).⁴¹ Evolutionary Atheism denies liberty,⁴² or alleges mutability leads to error,⁴³ and so eliminates God, but St. Thomas answers: "Likewise acts of the free will must be led to a cause higher than the reason and will of man, because these are mutable and full of defects, and everything mutable, and those things that are able to become less, must be reduced to an immutable prime cause which is per se necessary" (Aquinas Summa Theologiae 1. 2. 3. ad 2).⁴⁴

Does St. Thomas assert that atheists can know the existence of God? Yes, anyone can know the existence of God by reasoning, arguing, and reflecting on things and man himself; but it is also true that the existence of God is not obvious since man does not have a vision or intuition of God.⁴⁵ The existence of God can be demonstrated easily, spontaneously, accessible to all, even to the uneducated, because there are many indications, traces, and phenomena that reveal God and demand His existence: such as the phenomenon of change, secondary causality, contingency, grades of being, order, participation, and composition of essence and existence.⁴⁶ If reason does not just glance at these things but attentively considers them, atheists can recognize God in these things.

Does St. Thomas argue for the existence of God? Yes, he does in five ways.⁴⁷ Several of these ways have particular relevance to evolution.⁴⁸ Evolution involves metaphysical motion, the first argument of St. Thomas. Evolution involves grades of perfection, the fourth argument of St. Thomas. Evolution involves finality, the fifth argument of St. Thomas.

The Five Ways (Quinque Viae) are five arguments by which St. Thomas proves the existence of God in his book, the Summary of Theology (Aquinas Summa Theologiae 1. 2. 3).⁴⁹ The principle upon which these arguments rest is the principle of causality, or immediate deductions from it. The first argument is from motion and concludes to an Unmoved Mover. The second argument is from essentially ordered causes (ex causis per se ordinatis) and concludes to a cause which is not subordinate in operation, which is the Prime Cause and Sufficient Being (ens a se). The third argument is from contingent (corruptibilibus) things and concludes to a Necessary Being (ens non corruptible), which is through itself (per se) or from itself (a se) non-contingent (incorruptibile). The fourth argument is from diverse grades of perfection, and concludes to a Maximum Perfection, which is the cause of all perfection outside of God. The fifth argument is from order in the world, and concludes to an Intelligent Designer (intelligentiam ordinatricem).

How does St. Thomas demonstrate, in the first way,⁵⁰ the existence of God as the Mover absolutely Unmoved from the fact of motion in the world? It is clear that in the world something is moved, and what is moved is contingent, since a necessary being is not moved.⁵¹ But, what is contingent demands a necessary being, since a contingent being has its sufficient reason in another being outside any series of causes.⁵² This necessary cause is the cause of all motion and itself is absolutely immobile. Therefore, from metaphysical motion is demonstrated the existence of God as the Mover absolutely Unmoved. This proof pertains to Evolutionism, since evolution involves metaphysical motion.

How does St. Thomas demonstrate, in the second way, the existence of God as the First and Independent Cause from the fact of subordinate causes or causes dependent in operation? We see many causes in the world which are dependent on very many conditions and pre-requisites in order to operate, and so are contingent causes in being (in esse).⁵³ This is because operation reveals being (operatio sequitur esse). Further, necessary being does not depend on anything. But, a contingent thing demands a necessary thing demands a necessary thing and a Prime Cause not dependent on anything. Therefore, from subordinate causes is demonstrated the existence of God as the First and Independent Cause.

How does St. Thomas demonstrate, in the third way, the existence of God as an Absolutely Necessary Being from corruptible beings? We see in the world very many things that are corruptible, and so are contingent, since a necessary being is not corruptible.⁵⁴ But, the contingent being demands a being absolutely necessary. Therefore, from corruptible beings is demonstrated the existence of God as an Absolutely Necessary Being.

How does St. Thomas demonstrate, in the fourth way, the existence of God as a Simply Infinite Being from the grades of perfection? We see in the world that there are perfections of life, substance, and being according to greater or lesser grades, and so the lesser grades are finite, and so contingent, since a necessary being is not able to be finite.⁵⁵ But, a contingent being demands a necessary being, which is simply infinite. Therefore, from the grades of greater or lesser perfection is demonstrated the existence of God as a Simply Infinite Being. This proof pertains to Evolutionism, since evolution involves, at least in some species, progress to greater perfection.

How does St. Thomas demonstrate, in the fifth way, the existence of God as Intelligent Designer (intelligentia ordinatrix) from order in the world? To anyone who looks around there appears a marvelous order in the world.⁵⁶ But, the work of ordering is the work of intelligence. Therefore, there exists an intelligence ordering the world. And Again: That intelligence is either created or uncreated. If uncreated, that is God. If created, it demands an uncreated cause, which is intelligent, and which is not any less than its intelligent effects. Therefore, from order in the world is demonstrated the existence of God as the Intelligent Designer of the world. This proof pertains to Evolutionism, since evolution involves some finality as one species becomes a new species.⁵⁷

Does St. Thomas explain that it is necessary for man to know God? Yes, St. Thomas says it is necessary (oportet).⁵⁸ St. Thomas teaches, "Just as creatures would be imperfect if created by God and would not return to God, so the creation of creatures would be imperfect unless the return to God would balance that creation...Whence it is necessary (oportet) that human intellects should know God in order that their knowledge would balance the procession of creatures from God" (Aquinas De Veritate 20. 4).

Does St. Thomas hold that God’s Providence must be part of the evolutionary process? Yes, implicitly, since St. Thomas holds that nothing happens in the universe by chance alone, much less a phenomenon so important as the origin or development of life. St. Thomas teaches that "Divine Providence is not opposed to contingent things subject to chance, or fortune, or human will" (Aquinas Summa Contra Gentiles 3. 75). Everything is the fruit of the power and the wise actions of God.⁵⁹

Does St. Thomas illustrate the Anthropic Principle? Yes, St. Thomas teaches God’s goodness has ordered creation in the service of man.⁶⁰ St. Thomas says: "Creatures themselves tend to divine goodness to be essentially assimilated in that goodness. But because the best way to be assimilated is to become like the thing that is better, every corporal creature tends to be assimilated to an intellectual creature as far as possible, in order to attain divine goodness in a higher way. Because of this, even the human form...is said to be the ultimate goal of lower nature" (Aquinas Scriptum in Liber Sententiarum 2. 1. 2. ad 3).

Can St. Thomas give a "sufficient reason" for evolution? Yes, St. Thomas implicitly does by dealing with "Communication," which is the spontaneous and generous giving of self by subsistent Esse.⁶¹ St. Thomas teaches: "Naturally existing things not only generally incline to their own good to seek it when they do not have it, and to repose when they possess it; but also to lavish in other as much as possible for them. So we see every agent, in the measure in which it has actuality and perfection, tends to produce things similar to itself. And thus is found in nature the desire to communicate to others, in the measure possible, the good possessed" (Aquinas Summa Theologiae 1. 19. 2).⁶² Since goodness by its very nature is diffusive, beneficial, the Scholastic philosophers number it among the primary principles, teaching: "Goodness spreads itself" (Bonum est diffusivum sui).⁶³ This is a prime principle both in the area of final causality and in the area of efficient causality.⁶⁴ In the area of final causality, it means that the end communicates its goodness to the means, e.g. a bitter drink becomes good if it is medicine for health (Aquinas De Veritate 21. 1. ad 4). In the area of efficient causality, it means that goodness produces other beings which participate in its goodness (Aquinas De Potentia 3. 6; Aquinas Summa Contra Gentiles 3. 24). As a final cause in evolution, the new and superior species would make its production good even if the old species ceases; and as an efficient cause in evolution the old species would share its goodness with a new species.