A masterpiece and must-read for those with an interest in the philosophical concept of God through the Ages. Etienne Gilson shines in this timeless classic and highlights his contribution as a historian of philosophy and prominence among the Thomistic elite. The intellectual depth and clarity embedded in this slim text of 144 pages is profound. It’s divided into four sections: God and Greek Philosophy, God and Christian Philosophy, God and Modern Philosophy, God and Contemporary Thought.
The Greek Philosophy section celebrates the shift from mythology to philosophy to “the God of the Philosophers”. He brings forward Plato’s realm of Ideas, the Forms, timeless and eternal truths, which we tend to conceptualize as the ultimate reality, separate from matter and sense perception. To Plato, the material reality is always changing; so truth could only come in the timeless and eternal realm of Ideas, “the Good” of which is the ultimate Idea. At this point, there is usually an immediate breakout of timeless metaphysical debate – between rationalism and empiricism, theism and materialism – but this is not Gilson’s aim. Gilson’s endeavor is a historical and metaphilosophical one on how the God of religion is informed by philosophy and vice versa.
He certainly gives credence that “the Good” is the ultimate Idea and that “nothing more closely resembles the definition of the Christian God than this definition of the Good” (page 25), but is quick to point out some problems, namely “that in Plato’s mind, the gods were inferior to the Ideas” (page 27), rendering “his gods distinct from his philosophical principles” (page 29), and reminding us that “Plato, who seems to have invented Ideas as a philosophical principle of explanation, did not invent the gods” (page 30).
For Plato, there are gods, Ideas, and “the Good” (the supreme idea) but in shifting to Aristotle, we find an “epoch-making event in the history of natural theology is that in it the long delayed conjunction of the first philosophical principle with the notion of god became an accomplished fact. The prime mover of the Aristotelian universe is also its supreme god” (page 32). The impact of this shift cannot be understated and impart three distinct developments:
1) Religion (by way of myth and tradition)
2) The birth of philosophy to explain the ultimate nature of reality (by way of open criticism of ideas)
3) The birth of philosophical theology (by way of reasoning to God as the first principle)
In Gilson’s words, “with Aristotle, the Greeks had gained an indisputably rational theology, but they had lost their religion” (page 34).
After the birth of philosophical theology, we now enter the second section on God and Christian Philosophy, of which we now approach the revealed God of the Old Testament, “I AM WHO I AM”, and in Gilson’s view, another paradigm:
“Here again historians of philosophy find themselves confronted with this to them always unpalatable fact: a nonphilsophical statement which has since become an epoch-making statement in the history of philosophy. The Jewish genius was not a philosophical genius; it was a religious one. Just as the Greeks are our masters in philosophy, the Jews are our masters in religion.”(page 40-41)
The revealed God, “I AM”, is now superimposed upon the philosophical first principle, namely being. God then is pure Act, Being itself, “the only possible explanation for the presence of such finite and contingent beings” (page 53). We now embark upon the genius of Aquinas on the value of faith and reason, upon his “straight metaphysical knowledge, where he says that ‘all knowing beings implicitly know God in any and everything they know’, though at the same time, “impossible to go further, because human reason cannot go further than the highest of all metaphysical principles” (page 78).
If Aquinas’ contribution to philosophical theology was profound, what then of Descartes? To Gilson’s words, “what was new with Descartes was his actual and practical separation of philosophical wisdom and theological wisdom. Whereas Thomas Aquinas distinguished in order to unite, Descartes divided to separate” (page 77). Gilson takes issue at Descartes’ method, in this separation, and also in pretending to be skeptic (even of one’s own existence) while maintaining both Christian belief and rationalism, the idea of God innately in our intellect. Gilson maintains “the only context in which the metaphysical conclusions of Descartes made sense was the metaphysics of Saint Thomas Aquinas” (page xxiv). Aquinas, following Aristotle, was an empiricist, reasoning from experience to causes.
“It is here where Gilson makes a key point that “even as a philosophical supreme cause, the God of Descartes of God is stillborn God… reduced to the condition of philosophical principle… an infelicitous hybrid of religious faith and rational thought… a God was that his creative function had integrally absorbed his essence…. no longer ‘He who is’ but rather ‘The Author of Nature’. Assuredly, the God of Christianity had always been infinitely more than that, whereas, after Descartes, he was destined progressively to become nothing else than that” (page 88-89). Descartes’ mechanistic conception of God, in Gilson’s mind, becomes nothing more than the “Author of Nature”, “watchmaker” or intelligent designer in the clouds, ultimately Plato’s Demiurge, except “this time, before beginning to arrange the world, the Demiurge had consulted Newton” (page 107). Unsurprisingly, Gilson rejects such a conception, “but the fact that there is no Demiurge does not prove that there is no God” (page 108).
The final section, God and Contemporary Thought, focusses on “the Criticism of Kant and Positivism of Comte” (page 109). But to the question of a robust Christian metaphysics, Gilson dismisses Descartes method as inconsequential and Kant’s as the worthy opponent of Aquinas:
“Today our choice is not Kant or Descartes; it is rather Kant or Thomas Aquinas. All other positions are but halfway houses which lead either to absolute religious agnosticism or the natural theology of Christian metaphysics. Philosophical halfway houses are pretty crowded, but never more than they are in our own times, especially in the field of natural theology. This fact is not a wholly inexplicable one. What makes it difficult to go back to Thomas Aquinas is Kant” (page 114).
The genius of Kant, like Hume, is that his philosophical challenge cannot be ignored, it must be engaged. Perhaps more powerfully stated by James Brent: “henceforth, any attempt to do classical theology, natural theology, or metaphysics had to answer the Kantian challenge.” 
Perhaps, like Hume, we engage such thoughts naturally, Hume in our skeptical moments, and Kant in our transcendental ones. Kant’s transcendental idealism separates phenomena, things as we perceive them, from noumena, “things in themselves” (of which we cannot have knowledge). Kant’s shift is from the primacy of metaphysics to a critical knowledge, limiting knowledge as to what can be perceived. At first blush, our Kantian hairs tingle at what seems an obvious truth.
The heart of Kant’s challenge is this: how can we have any knowledge of God? If God is a “thing-in-itself” of which we cannot perceive, there is no natural theology. Contra Aquinas, with Kant, God “cannot be related to anything else by the category of causality… he is not an object of cognition” (page 111). “God exists” would not be a position of reason as our reason cannot extend into such a “thing in itself”. God can certainly be posited to ground morality or “the unifying power of human reason” (page 118), but can only be made in faith (and by the way required to make such a ground).
Something has gone awry, yet Kant certainly succeeds in giving us pause. Do we really perceive any object in itself? We only perceive sensations as given to us by the limits of our perception. We return to one of Gilson’s main points – the choice between Kant and Aquinas. For Aquinas, a realist, the picture is much different. True, there are limits to our reason, but we can reason, albeit finitely, towards causes, God as the Final Cause – pure Act. In Gilson’s words:
“Metaphysics posits God as a pure Act of existence, but it does not provide us with any concept of His essence. We know that He is; we do not comprehend Him… Being men, we can affirm God only on anthropomorphic grounds, but this does not oblige us to posit Him as an anthropomorphic God.” (page 141-142).
We arrive back to the limits of our reason and possibility of any knowledge of God. God, pure Act, whose very essence is his existence, obviously surpasses our understanding but we’re reminded again of the genius of Aquinas, that reason and faith both reach towards understanding, reason being a “preamble” to faith. In Aquinas’ own words:
“The existence of God and other like truths about God, which can be known by natural reason, are not articles of faith, but are preambles to the articles; for faith presupposes natural knowledge…”
Reason, having its limits (not so controversial), and being a preamble to faith speaks not only to a complementary relationship between reason and faith (quite controversial) but also an eventual shift from reason to faith.
There are of course options, including the outright rejection of faith, or requiring faith alone to be the sole method to seek God, but for the faithful inspired by metaphysical approach; they can find God in a limited sense through the light of human reason, while realizing this is a far as it goes. The “leap of faith”, then, is not based on blind fideism, but a faith decision made from a place of reason.
For some, the ascent from the God of the philosophers to the God of Abraham seems entirely unjustified. For others, the revealed “I AM” establishes not only “He Who Is”, Pure Act, Being and Goodness itself, the Final Cause to which all things aim, but also establishes the role for reason in the knowledge of God (God’s revelation to Moses building upon the preamble of reason already conceived). God then, can be sought by the philosopher and the theologian alike. In summary, and much more eloquently stated by Gilson:
“Where a man’s metaphysics comes to an end, his religion begins. But the only path which can lead him to the point where the true religion begins must of necessity lead him beyond the contemplation of essences, up to the very mystery of existence. This path is not very hard to find, but few are those who dare to follow it to the end. Seduced as they are by the intelligible beauty of science, many men lose all taste for metaphysics and religion. A few others, absorbed in the contemplation of some supreme cause, become aware that metaphysics and religion should ultimately meet, but they cannot tell how or where; hence they separate religion from philosophy, or else they renounce religion for philosophy, if they do not, like Pascal, renounce philosophy for religion. Why should we not keep the truth, and keep it whole? It can be done. But only those who realize that He Who is the God of the philosophers is HE WHO IS, the God of Abraham, of Isaac, and of Jacob.” (page 143-144).
 Thomas Aquinas, Summa Theologica, Ia, q. 2, a. 2