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.