I shall address Plantinga's A/C model. He says "anything on which Aquinas and Calvin are in agreement is something to which we had better pay careful attention". I cannot claim to have any meaningful knowledge of Calvin, but I thought that I would explore the ways in which Aquinas and Plantinga's model differ. It seems to me that the role of the Holy Spirit is different in Aquinas and Plantinga. In addition, the means by which we know God is different Aquinas does not have a faculty especially for the knowledge of God as it seems Plantinga does.
First, a brief synopsis of Plantinga's A/C model: we have a sort of innate idea, or disposition to form beliefs about God in various circumstances. Plantinga calls this the sensus divinitatis. Conditions of natural beauty, for instance, serve as occasions on which beliefs about God arise. These beliefs are not conclusions, for which the experience serves as evidence or premise. Instead, the belief about God in these circumstances is similar to a perceptual belief, like the belief that the tiger lilies are in bloom. One does not draw an inference from appearances, which leads to the conclusion that the tiger lilies are in bloom. One simply sees that the tiger lilies are in bloom. Both the perceptual belief and the belief in God are basic, in the sense that these beliefs are not accepted on the basis of other beliefs.
How, then, can anyone fail to know God? It seems that, according to Plantinga's A/C model, failing to know God is like failing to see that the tiger lilies are in bloom. However, spiritual blindness, so to speak, seems far more common than physical blindness. People's failure to reap the benefits of the sensus divinitatis is the result of sin. Sin causes both a failure of intellect one is unable to see the glory of God and a failure of will one loves and hates the wrong things. Faith requires not only knowledge of God, but also love for God. Both knowledge and love occur at the instigation of the Holy Spirit.
Now a brief summary of Aquinas, which I'll go on to elaborate. Aquinas does not think that there is any sensus divinitatis; we know God with the same faculties with which we know anything else. Aquinas also distinguishes between natural and revealed theology, and many truths for example that God exists or is infinite can be known in the same way that we know any philosophical truth. Revealed truth is also required for salvation. Sin harms human nature in Aquinas, of course, but it does not harm natural reason at all, and harms natural love of God only partially. The Holy Spirit is required for some knowledge of and love of God.
First, let's consider natural and revealed theology. By reason alone we can know that God exists, is infinite, good, immutable, eternal, and other such traditional ascriptions. In fact we can know of God whatever is necessary to account for the being of the things of sense that we know. There are other things that cannot be known by reason alone the mystery of the incarnation, or the sacraments of the church. As Aquinas says, "it was necessary for the salvation of humans that certain truths which exceed human reason should be made known ... by divine revelation" [ST I 1 1]. But also, even the truths that can be known by natural reason are revealed. What can be known of God through natural reason takes a long time, and can be known only by a few. Not everyone has the opportunity or intellect for philosophy. But since every person should have the means for salvation, and know the truths that are required, along with "philosophical science built up by reason" there must be revelation. Not everyone knows God in the same way, and a model of how faith works cannot concentrate on only one means. Some people may approach God through philosophy; some people through scripture; some people through observing the happiness of others.
However, humans can know God with the same faculties with which they know anything else. Aquinas does not think that we require a different faculty especially for knowledge about God the sensus divinitatis is not a part of Aquinas's account of faith. Humans have sensation and reason. The same rational faculty that permits any philosophical knowledge can also be turned to philosophical theology. In knowing the truths of natural theology, we begin with sense (e.g. "It is certain, and evident to our senses, that in the world some things are in motion" [ST I 2 3].) Thereby we move via reasoning to the principles of these sensory things, and demonstrate that God exists. Even in natural knowledge, the Holy Spirit may improve these natural faculties make the natural light brighter, as it were.
Even in cases of supernatural knowledge, the faculties are the same as they are in natural scientific or philosophical knowledge. The faculties may be used on non-natural objects. So, for instance, we can receive images directly, which express divine truths better than the images we receive directly from sensible things, as St Ignatius Loyola received a vision of three keys (of a musical instrument) playing a single chord, expressing the Trinity. We can sometimes have sensory knowledge of non-natural things, for example voices and images, as in the case where the Holy Spirit was seen in the shape of a dove and a voice was heard saying This is my beloved son. In all these cases, the normal human knowledge-apparatus is what's doing the work, and the Holy Spirit is aiding it. [ST I 12 13]
We need the grace of God to know truth, and every truth is from the Holy Spirit. But this is ambiguous, and means something different depending whether we're talking about natural or supernatural truth. On the one hand, we can only do anything at all by the grace of God, since God created us with any abilities we may have. On the other hand, in some cases some extra grace is required. God created humans with the ability to sense and to reason. (In addition, God is the first mover, so in Aquinas's cosmology God is also required as an ultimate efficient cause, in order that anything at all happen.) This does not mean, however, that in natural theology God is supernaturally involved in individual acts. Humans can know any of the things that begin with sense, with nothing more than the faculties given by God in the first place. Beginning with sense can lead to a conclusion about God, as in the five ways. In this sense, saying that we need the grace of God in order to know natural truth means that without God we would not exist, would not have the ability to sense or to know or to sustain any human activities. In order to know supernatural truth, however, humans need extra grace added, and in that case, to say that we could not know without the grace of God is to say that we could not know any particular truth without a separate act of God to confer the material of knowledge. [ST I-II 109 1]
This view of theological knowledge is consonant with a different model of knowledge in general. Just as Wittgenstein thinks that there can be no sense in a private meaning, but only in a publicly shared and publicly validated meaning, in a community of speakers and knowers, so Aquinas's model of revealed truth requires publicly shared and publicly validated theological knowledge in a faith community.
Tradition is a guide to both naturally known and revealed truth. When Aquinas says that philosophical theology is built up by natural reason, he does not mean that it is built up by each individual's natural reason. Descartes was truly innovative in deciding that he could know all truth from the beginning by himself. Aquinas has Aristotle, Augustine, Maimonides, Ibn Sina, St Paul, and numerous other authorities upon whose arguments he can build. Often this building consists of identifying errors; but the presentation of the erroneous argument permits the identification of the errors and the growth of knowledge that results. Additionally, since any personal revelation must be interpreted, it must be interpreted in accordance with the truth revealed by the prophets and disseminated by the Church. In other words, revealed knowledge does not require a sort of "built-in" warrant that relies upon the situation in which it arises.
I'll look briefly at two ways in which will is comprised in Aquinas's account. First, will is required for faith, in so far as humans need to assent to the articles of faith. Second, will is involved in love for God.
According to Aquinas, faith requires that certain propositions be proposed to humans, either directly as to the apostles and prophets, or, for most of us, indirectly via these apostles and prophets and other people whom God has sent. In addition, faith requires assent to these propositions. This assent is an act of will. The situation is similar for the will as for the intellect. So, just as in natural knowledge, the faculties that are part of human nature are sufficient to discover natural truth, the will is sufficient to give assent to natural truth. However, as something must be added to the intellect in order to know supernatural things, similarly the will requires grace in order that humans give assent to supernatural things. The will must also be raised by God to assent to the things that are above human nature. [ST II-II 6 21]
Aquinas says that faith is part way between science and opinion. By science, he means knowledge widely, including philosophy, not only what we would call science essentially, propositions for which we have demonstrations. He says that in the case of science, one is moved by the object to give assent; one sees (with the senses or the intellect) that the proposition is true. In this case, you assent to the proposition with understanding of the principles, as it were, that lie behind it. In the case of opinion or faith, one assents to the proposition through an act of choice. If the assent is accompanied by doubt and fear of the other side, one has opinion. If the assent is given with certainty, and no fear of the other side, one has faith. However, one does not see, with the senses or the intellect, the things of faith or opinion. [ST I-II 67 3; II-II 1 4]
As I said above, some of the same propositions can be a matter of faith for one person and a matter of knowledge for another, depending on one's habits of mind. But some propositions are necessarily a matter of faith, and cannot be for anyone a matter of science. Moreover, the proofs or demonstrations e.g. of God's existence do not prove the entire Christian God with all its traditional properties to say that there is a first mover leaves open the possibility that that first mover is a collection of natural laws.
To what extent does sin compromise the natural love of God? It is natural to all things to love God above everything else. Humans in the state of perfect nature would love God above all things, without the help of any additional grace. However, in the corrupt state, humans need grace in order to heal their nature and to love God above all things.
There are three goods of human nature, as Aquinas calls them, that could be damaged by sin. [ST I-II 85 1] First, the principles of nature the faculties of the soul, and so on. These goods were not diminished whatsoever by sin. Rational nature cannot be destroyed, or people would not be capable of sin. Second, the gift of original justice, which was completely destroyed. Finally, the inclination to virtue, which was diminished but not destroyed by sin. People still have the inclination to virtue, or they could have no remorse; they fail in carrying out the inclination. Humans can still do particular goods, "build dwellings, plant vineyards and the like" [ST I-II 109 2]. But they cannot do all the good natural to them they fall short of the good that they could do. In the corrupt state, humans can still will good and do good, but only private, personal good. They need grace to do better.
According to Aquinas, all things have a natural inborn love for God. Not only humans, but also plants and animals love God. St Thomas says "all things, by desiring their own perfection, desire God" [ST I 6 1 ad 2] That is because all things desire to preserve and augment their being, by attaining their natural ends. Desire does not necessarily mean conscious desire. E.g. a plant does not think that it wants to live and grow; rather it consistently acts in such a way as to seek water and light. Humans, similarly, desire happiness naturally, and consistently seek it. That is the sense in which a knowledge of God is innate, since whatever living things naturally desire, they must have a natural means of knowing. But even though humans naturally desire happiness, and their ultimate happiness is in fact to love God, they do not necessarily find themselves desiring to love God. They may not know God explicitly. Some people, says Aquinas, think that their happiness is to be found in riches or pleasures. Anything has to desire what is good or it would die but humans can desire many different kinds of goods without desiring the ultimate good, or explicitly realizing that God is the ultimate good. [ST I 2 1 ad 1] (In the case of plants and animals, seeking the particular good is their means of loving God.)
Plantinga differs from Aquinas in the means by which we know God. In
Aquinas, the Holy Spirit is required only for supernatural knowledge. Not
all knowledge of God is supernatural for Aquinas, as it seems to be for
Plantinga. Plantinga's sensus divinitatis appears to be a supernatural