Saint Thomas Aquinas wrote his best-known work, the Summa Theologica, as a compendium of Catholic teachings for theology students. Its readers are sometimes surprised to find very little in this massive work that follows the path of modern Christian apologetics, since it was written for those who were already Catholics. The justifications that are lacking in the Summa Theologica are to be found in the Summa contra Gentiles, which Aquinas wrote specifically to aid in the conversion of Jews and Muslims.
The overarching argument of the text, therefore, is that Christianity is is the true faith, and that any any dissenting points of doctrine in Islam and Judaism are in error. The arguments are directed at other monotheists, so not much time is spent on the question of God's actual existence, though this is briefly addressed in book 1, which argues that a reasonable person ought to accept...
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that there is one God before proceeding to a much more detailed discussion of what the attributes of that God would be.
Book 2 argues that creation requires a creator, while book 3 examines the relationship between the creator and his creation. Finally, book 4 looks at specific doctrines, such as the Trinity, which separate Christianity from other monotheistic religions, and provides technical justifications for the Christian position.
What primary concepts does Summa Contra Gentiles use to make its arguments?
Summa contra Gentiles was written to assist in the conversion of Jews and Muslims. It is, therefore, more argumentative and apologetic than Saint Thomas Aquinas's other works. However, the text does not spend much time or effort in justifying the existence of God, as its targets are not atheists. An important primary concept, therefore, is that of monotheism, on which Christians, Jews, and Muslims can agree.
Saint Thomas's argument, however, is that we have no primary concept of God in the way that the term is usually understood. When we talk about objects in nature, we have primary concepts such as "dog" or "tree" or "water" which allow everyone to be sure they are discussing approximately the same thing (even though individual dogs and trees are quite different). This is not the case with God. A discussion of God uses philosophy to establish his attributes. Apart from the idea that there is only one of him, Aquinas argues for the following foundational properties of God: He is eternal; he is something other than the matter of the universe (i.e., pantheism is incorrect); he is not a body and is not distinct from his essence (as, for instance, individual human beings are distinct from the concept of humanity); he is perfect; and he is unlimited. From these foundational concepts, Aquinas goes on to make more complex claims about the nature of God and the truth of Christianity.