Summa Theologica by Thomas Aquinas
A monumental theological work systematically exploring Christian doctrine. Aquinas addresses topics such as the nature of God, morality, sin, and salvation using philosophical reasoning, blending faith and logic in a comprehensive guide to medieval Christian theology.

Thomas Aquinas (1225–1274) was a medieval philosopher and theologian, best known for Summa Theologica. He integrated Aristotelian philosophy with Christian theology, shaping Catholic doctrine. His work on natural law, ethics, and metaphysics remains foundational in Christian philosophy and Scholasticism.