The Summa Theologiae or Summa Theologica (transl. ’Summary of Theology’), often referred to simply as the Summa, is the best-known work of Thomas Aquinas (1225–1274), a scholastic theologian and Doctor of the Church. It is a compendium of all of the main theological teachings of the Catholic Church, intended to be an instructional guide for theology students, including seminarians and the literate laity. Presenting the reasoning for almost all points of Christian theology in the West, topics of the Summa follow the following cycle: God; Creation, Man; Man’s purpose; Christ; the Sacraments; and back to God.
Although unfinished, it is “one of the classics of the history of philosophy and one of the most influential works of Western literature.” It remains Aquinas’ “most perfect work, the fruit of his mature years, in which the thought of his whole life is condensed”. (SOURCE: Summa Theologica – Wikipedia)