Summa contra Gentiles

The central framework is the distinction between the preambles of faith—truths that natural reason can establish, such as God's existence, unity, and goodness—and the articles of faith—truths known only by revelation, such as the Trinity and Incarnation. The work aims to equip Christians to engage non-Christians, particularly Muslims, with reasoned argument, showing that the fundamental truths of theism are accessible to reason apart from revelation . Book I treats God as He is in Himself; Book II treats God as the origin of creatures; Book III treats God as the end of creatures; Book IV treats the truths that surpass reason. The work is often described as the first systematic natural theology in the Christian tradition, though it is also much more than that.

The title suggests a polemic against non-Christians, but the work is not primarily an attack. Aquinas's method is to argue from premises that his non-Christian interlocutors would accept, relying on reason and on authorities shared with Jews and Muslims rather than on distinctively Christian scripture . The work was traditionally thought to have been written for the Dominican mission in Spain to convert Muslims, but modern scholarship has questioned this. Some scholars argue that the title was not Aquinas's own and that the work was addressed primarily to Latin Christians, not to unbelievers . The Summa contra Gentiles is better understood as a work of Christian self-understanding, showing how the faith is rationally defensible, than as a missionary manual.

The two works are complementary but distinct in purpose and audience. The Summa contra Gentiles was begun around 1259 and completed around 1265; the Summa Theologica was begun around 1265 and left incomplete at Aquinas's death in 1273 . The Summa contra Gentiles is more philosophical and apologetic, addressing non-Christians and arguing from reason to the preambles of faith. The Summa Theologica is more pedagogical and theological, designed for beginners in theology and organized around the exitus-reditus framework . The earlier work is often more speculative and experimental, while the later work is more systematic. Readers who want Aquinas's natural theology often turn to the Summa contra Gentiles; readers who want his full systematic theology turn to the Summa Theologica.

The work was composed over several years, probably between 1259 and 1265, and was first printed in 1485 by Peter Schöffer in Mainz, the same printer who issued the first edition of the Summa Theologica . The Leonine critical edition was published in 1918. The work survives in numerous manuscripts, and its composition history has been debated: some scholars have argued that it was written in stages, with Book IV added later, while others see it as a unified project. A distinctive feature of the textual tradition is that the work includes many quotations from Islamic and Jewish philosophers, including Averroes and Maimonides, which were studied carefully by later readers. The work's influence on later natural theology, from the early modern period to the present, has been immense.

The Summa contra Gentiles is notable for its engagement with Islamic thought: Aquinas cites Averroes and Maimonides extensively and argues against positions he attributes to Muslim philosophers, particularly on the eternity of the world and the nature of the soul . But the work's knowledge of Islam is limited and mediated by Latin translations and secondary sources. Aquinas does not engage the Qur'an directly, and his treatment of Islamic theology is often superficial. Modern scholars have criticized the work for its polemical framing and its failure to engage Islam on its own terms. A more positive assessment notes that the work represents a serious attempt to argue rationally with non-Christians, and that its method of finding common ground in reason remains a model for interreligious dialogue.

The most common misunderstanding is that it is a manual for converting Muslims, or that it is a polemic against non-Christians. It is neither. The work's method is rational argument from premises that non-Christians can accept, and its audience is as much Christian as non-Christian: it aims to show Christians how to defend their faith rationally, and to show non-Christians that the fundamental truths of theism are accessible to reason. A related misunderstanding is that the work is purely philosophical, with no appeal to revelation. In fact, Book IV is entirely devoted to truths that surpass reason, and the work is explicitly a work of Christian theology, not of neutral philosophy. The title is misleading; the work is a work of Christian self-understanding through rational argument.