St. Thomas Aquinas 
Summa Contra Gentiles, 4 
Book Four: Salvation

Book Four of the Summa Contra Gentiles examines what God has revealed through scripture, specifically the Trinity, the Incarnation, and the end of the world.The Summa Contra Gentiles is not merely the only complete summary of Christian doctrine that St. Thomas has written, but also a creative and even revolutionary work of Christian apologetics composed at the precise moment when Christian thought needed to be intellectually creative in order to master and assimilate the intelligence and wisdom of the Greeks and the Arabs. In the Summa Aquinas works to save and purify the thought of the Greeks and the Arabs in the higher light of Christian Revelation, confident that all that had been rational in the ancient philosophers and their followers would become more rational within Christianity.Book 1 of the Summa deals with God; Book 2, Creation; and Book 3, Providence.