Summa Theologica is a lengthy religious treatise by Thomas Aquinas, a Franciscan friar who completed this vast tract while teaching at the University of Paris. It is a longer, more comprehensive work of his Summa Contra Gentiles. Aquinas wrote during the High Middle Ages and his treatise was intended for those new to studying Christianity.

It is unique in bringing together Church doctrine and philosophy. It is divided into three parts, but the first part is divided into two parts, and the third part has a supplement, with the result that there are five de facto sections of varying lengths, arranged by subject. Each part is in turn divided into several articles. The articles are phrased as Socratic-style questions, which have objections submitted in reply to them in order to arrive at a conclusion on each point.

The major principles treated and conclusions reached in the first part of Thomas Aquinas' Summa is that God is coeternal with the Son and Holy Ghost, and that man is endowed with a soul that is immaterial and unique to each individual.

His second part address seven virtues (three theological and four cardinal): Faith, Hope, Charity, Prudence, Justice, Fortitude, and Temperance. He expounds on each of these, their opposite vices, and how they are featured in acts of martyrdom.

The third part talks about the sacraments, including Baptism and Confirmation. To Aquinas the sacraments are not symbolic, they impart grace in their practitioners.

His appendix discusses souls after death (on which he states that souls go to different places depending on their level of nobility), institutions of marriage (a cure of concupiscence), and indulgences (which are not sufficient to redeem sin).

Aquinas quotes heavily from Scripture, and also asserts that sacred doctrine is not a science, as it requires Faith, and cannot be fully tested.