Summa Theologica

St Thomas Aquinas with the Summa, painting by Benozzo Gozzoli

The Summa Theologica is a massive work by St Thomas Aquinas, a Dominican priest whose feast is celebrated today in the liturgical calendar. It is a Theological compendium, written between 1265 and 1274, and although unfinished, it is one of the classics of the history of philosophy and one of the most influential works of Western literature. It is intended as a manual for beginners in theology and a compendium of all of the main theological teachings of the Church. It presents the reasoning for almost all points of Christian theology in the West. The Summa's topics follow a cycle: the existence of God; Creation, Man; Man's purpose; Christ; the Sacraments; and back to God. 

The Summa, printed in 1463 by Johann Mentelin

The Summa is composed of three major parts, each of which deals with a major subsection of Christian theology. The First Part deals with God's existence and nature; the creation of the world; angels; the nature of man. The Second Part deals with general principles of ethics and morality, including individual virtues and vices. The Third Part treats the person and work of Christ, who is the way of man to God; the sacraments; the end of the world. Aquinas left this part unfinished.
St Thomas Aquinas was born in 1225 at Roccasecca and died in 1274 at Fossanova, both in Italy, and was an immensely influential philosopher and theologian in the tradition of scholasticism. He was known as Doctor Angelicus, the Angelic Doctor.