Summa contra Gentiles

The Summa contra Gentiles (also known as Liber de veritate catholicae fidei contra errores infidelium, "Book on the truth of the Catholic faith against the errors of the unbelievers") is one of the best-known treatises by Thomas Aquinas, written as four books between 1259 and 1265.

Whereas the Summa Theologiæ was written to explain the Christian faith to theology students, the Summa contra Gentiles is more apologetic in tone. While the last book deals with topics of revealed theology, such as the trinity, incarnation, and the sacraments, the remaining three books limit themselves to Natural Theology: Arguments based on reason, Thomas believes, will also be understood also by those who do not believe in Christian revelation.

I have set myself the task of making known, as far as my limited powers will allow, the truth that the Catholic faith professes, and of setting aside the errors that are opposed to it. To use the words of Hilary: 'I am aware that I owe this to God as the chief duty of my life, that my every word and sense may speak of Him' (De Trinitate I, 37) 

The Summa contra Gentiles consists of four books. The structure of St. Thomas's work is designed to proceed from general philosophical arguments for monotheism, to which Muslims and Jews are likely to consent even within their respective religious traditions, before progressing to the discussion of specifically Christian doctrine.

Books I–III cover truths that naturally are accessible to the human intellect, while Book IV covers revealed truths for which natural reason is inadequate.