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 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.