Summa Theologica Qq 6 54

"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 Christianity in general and the Catholic faith in particular. The Summa's topics follow a cycle: the existence of God, God's creation, Man, Man's purpose, Christ, the Sacraments, and back to God. It is famous for its five arguments for the existence of God, the quinquae viae (Latin: five ways). Throughout his work, Aquinas cites Augustine, Aristotle, and other Christian, Jewish and even Muslim and ancient pagan scholars. The Summa Theologica is a more mature and structured version of Aquinas's earlier Summa Contra Gentiles. This earlier work was more apologetic, each article refuting a belief of a heresy. Aquinas's death left the Summa, perhaps the greatest theological statement of the Middle Ages, unfinished." (Quote from wikipedia.org)