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The Summa contra Gentiles (hereafter referred to as SCG) was written by St. Thomas Aquinas between 1258 and 1264. The work has occasioned much debate as to its purpose, its intended audience, and its relationship to his other works. Thomas' work is divided into several categories: Scriptural commentaries, Aristotelian commentaries, Opuscula (or smaller works), disputed questions and theological syntheses. The SCG is usually classified as a theological synthesis along with his earlier Commentary on the 'Sentences' of Peter Lombard and his well-known Summa Theologiae, although there are significant differences in scope and intent between all three of these works.

The SCG has an odd structure in comparison with Thomas' other theological syntheses and other scholastic literature as well. It is split roughly into two sections, books I-III (which cover truths that naturally are accessible to the human intellect), and book IV (which treats truths for which natural reason is incompetent (like the Trinity, Incarnation, Sacraments, and the Resurrection). More specifically, the first part of the work treats truths about God that are known by the natural capacities of the human intellect. Thomas posits that we can know that God exists, that God is one, that God is good from the power of unaided reason. Each of the three first books embodies a different way of exploring humanity's natural knowledge of God. Book I treats God in himself (his knowing and willing). Book II treats God in his so-called transitive action (action that goes out from him) and thus is a study of creation. Book III shows how, for Thomas, all created things have their end in God. Book IV can be seen to mirror the basic structure of books I-III, although it treats the issues from the perspective of revelation. Thus the first part of book IV treats God in himself (the Trinity), then God in his transitive action (the Incarnation and Sacraments), and then God as the end of all things (the Resurrection).

The structure of the work has caused some controversy. Some Christians see an unnecessary division between divine truths and human truths. Thomas asserts, however, that the twofold division is solely due to the condition of human knowledge. In itself there is one truth, God's knowledge of himself. Humans, since we rely necessarily on the senses for natural knowledge, have two kinds of knowledge about God. One is exemplified by the gentiles' knowledge of God (what can be known by natural reason) and the other, those things that can only be known in the light of Christian revelation. Since Thomas treats what can be known about God in this bifurcated manner, many have called the SCG (at least the first three books) his most philosophical work, insofar as we would understand that term today.

Until the early 20th century there was a good consensus about why Thomas wrote it. A legend had been passed down saying that the head of the Dominicans, Thomas' religious order, Raymond of Penafort, had asked Thomas "to compose a work against the errors of the infidels, by which both the cloud of darkness might be dispelled and the teaching of the true Sun be made manifest." From this legend, most understood that the work had a missionary or apologetic intent. Thomas famously says in the prologue to the SCG that in debating Christian heretics one can make recourse to the entire Bible to show them their error. If one is debating with the Jews, then one can use the Old Testament as a basis of understanding. With Muslims and pagans there is no such recourse to a common authoritative text and one must then "have recourse to natural reason, to which all men are forced to give their assent" (SCG I, chapter 2). It was thus seen that the SCG was written to show unbelievers certain philosophically compelling arguments, arguments to which all are forced to give their assent, in order to prepare their way to assent to the Christian faith. Thomas clearly states that the revealed truths of Christianity cannot have the same recourse to reason. To try to argue for things like the Trinity and Incarnation within the limits of natural reason would open Christianity to ridicule, since obviously Christians do not believe by such weak, rationalistic arguments.

Many today doubt the veracity of the legend regarding Raymond of Penafort, and even if the legend is accepted, that Thomas intended the work as a missionary manual either to be put into the hands of unbelievers so that they might read it, be convinced of the truth, and convert or as a training manual for Dominicans to learn how to argue for the truth of the faith. The arguments are many and convincing. One of the more convincing arguments states that missionary manuals were a known form of Dominican writing, for which there were certain standards of knowledge of the opposing point of view. Thomas only makes a handful of references to Muslims, the putative audience, and even once states his relative ignorance as to their teaching. Due to the standards of the day it would not have made a good missionary manual.

Another point to be made goes beyond the historical contingencies that may have occasioned its writing to what the text actually treats. Some of the earliest manuscripts have a different title than 'Summa contra Gentiles'. It is also called "The Book on the Truth of the Catholic Faith against the Errors of the Infidels." There is a significant difference between a book and a 'summa'. The less well known title would make the work a book primarily about truth, albeit the truth of the Catholic faith. It is more common today to see the work as a treatise on truth, how humans come to know the truth of material things which then lead to knowledge of the divine.