Theologians and philosophers who discuss the doctrine of natural knowledge of God in a classroom setting will sometimes conflate the different versions of the cosmological argument without realizing that, although some versions of the argument are sound, others are susceptible to criticisms. With this realization in mind, John Knasas emphasizes the importance of distinguishing the various God proofs, favoring Aquinas’ natural theology over Leibniz's contingency argument for the existence of God: “Leibniz's thinking about existence in these proofs is … remarkably shallow. Hence, Kant and others could easily raise objections to the proofs. The remedy to that shallowness brings me to Thomistic Existentialism” (2). Privileging the act of being as central to Aquinas’ metaphysics, Thomistic existentialism was a dominant school of Thomism in the middle of the twentieth century. Etienne Gilson, Jacques Maritain, and Joseph Owens were some of the major pioneers of Thomistic existentialism.

One of the major themes of Knasas’ book is that Aquinas’ earlier and influential tract, De Ente et Essentia, provides the hermeneutical key for appropriately understanding the various God proofs in the Thomistic corpus. For instance, a naive reading of the Five Ways (as represented in the Summa theologiae) may not be convincing to modern readers, but a grasp of the metaphysical schema that informs the Ways will help them to be seen as convincing. Time and again, Knasas presses home the importance of De Ente reasoning for demonstrating how the proofs were understood by Aquinas and his contemporaries. This metaphysical vision was neglected by Leibniz and other modern proponents of the cosmological argument. Thus, teaching the Five Ways in the classroom cannot be reduced to exposing students to one part of the Summa theologiae (which is commonly found in anthologies in the philosophy of religion) without a previous presentation of De Ente reasoning.

Knasas commences his study with a discussion of Leibniz's contingency argument (which has become a paradigm for modern cosmological arguments) and the Kantian critique that devolved into issues of ontological reasoning and whether existence is a predicate (chapter 1). After exposing some problems related to Leibnizian cosmological reasoning, Knasas situates himself within current debates in Thomistic philosophy (chapters 2–4). In chapter 5, a series of objections is mentioned and answered in response to Aquinas’ natural theology (e.g., existence as an attribute, challenges related to sense realism, the possibility of an infinite regress, the problem of evil, ontotheology, etc.). Knasas assures the reader that “unlike Leibniz's notion of existence in his a posteriori proofs, Aquinas’ thesis of esse is sufficiently nuanced to provide in the De Ente cosmological reasoning that stands the test of the cosmological debate” (5). The remainder of the book is dedicated to expositing Aquinas’ proofs within their proper historical contexts.

Thomistic Existentialism and Cosmological Reasoning serves as another powerful example of the revival of Thomistic natural theology that is underway in philosophical circles in the Catholic Church. This revival can be traced back to John Paul II's pontificate and his call for the restoration of philosophy and faith. Although many published works on Aquinas are exclusively centered on his writings, Knasas shows how Thomistic philosophy can be put in dialogue with modern philosophies, which are often based on premises that Aquinas and the Catholic tradition would hesitate to fully embrace. Developing an awareness of the difference between these paradigms is not a trivial matter in terms of the defense of Catholic belief. For the modern philosophical project is at the heart of the dialectics between atheism and theism.

Perhaps the one issue that is a cause for concern for some professors and their students is that the book is specialized for readers who are familiar with the intricacies of classical metaphysics and Thomistic philosophy. Unless the theologian is teaching a graduate-level course on arguments for the existence of God to precocious students, one must look elsewhere to introduce undergraduate students to the foundations of Catholicism.