Summa Theologica, the sixth edition (considered the "definitive" edition), in English translation

According to Wikipedia: "The Summa Theologica (or the Summa Theologiae or simply the Summa, written 1265-1274) is the most famous work of Thomas Aquinas (c. 1225-1274) although it was never finished. It was intended as a manual for beginners as a compilation of all of the main theological teachings of that time. It summarizes the reasonings for almost all points of Christian theology in the West, which, before the Protestant Reformation, subsisted solely in the Roman Catholic Church."