Question: "What is the Summa Theologica?"
Answer: Written from 1265—1273, the Summa Theologica (or Summa Theologiae, or sometimes referred to as the Summa) is the name of the philosophical summary (summa) of the theology (theologica) of the Roman Catholic Church as presented and organized by Thomas Aquinas, 1225—1274.
Aquinas used Aristotelian philosophy as a framework for the Summa Theologica, believing Aristotle to be a friend of Christianity when many earlier thinkers had seen him as a foe. Aquinas, however, did reject Aristotle’s concept of a detached and distant God in favor of the God of the Bible who is intensely interested and knowable. The Summa Theologica was declared to be the official teaching of the Roman Catholic Church by Pope Leo XIII (1879).
The Summa Theologica is divided into three parts. The first part covers the nature of God, creation, angels, man, and divine government (sovereignty). The second part addresses ethics, habits, law, faith, wisdom, self-control, morality, prophecy, miracles, and the contemplative life. The third part instructs the reader on the doctrine of Christ including His incarnation, life, death, and resurrection. In the same section, Aquinas addresses the need for sacraments in the remission of sins. A majority of the third section is devoted to explaining the sacraments.
The Summa Theologica is still in print and available today. Of course, it is a significant source of information about Roman Catholic theology, but there will be much in it that Protestants can agree with regarding evidence for God’s existence, ethics, epistemology, faith and reason, and anthropology. There are many evangelicals today, especially apologists, who consider themselves Thomists, or those who follow the thinking of Thomas Aquinas. (Norman Geisler, who died in 2019, was perhaps the most prominent modern apologist to fit this description.)
Protestants and Roman Catholics disagree about the issue of justification—how a person is made right with God—and, indeed, this was the central issue of the Reformation. Protestants will find the most disagreement with Aquinas in the third section of the Summa Theologica. However, Protestants and Catholics agree on a lot, especially as compared to the beliefs of an increasingly secular culture.