Part I: The Existence and Nature of God
Aquinas begins with foundational questions about God’s existence. He presents five classical arguments (the “Five Ways”) for the existence of God, including the argument from motion, causation, contingency, degree, and teleology. He discusses God’s simplicity, perfection, goodness, and omnipotence, establishing the essential attributes of the divine nature.
Part I-II: The Moral Law and Human Action
This section explores the nature of human beings, their ultimate end (happiness or beatitude), and the role of virtue. Aquinas distinguishes between natural law (moral principles known through reason) and divine law (revealed in Scripture). He elaborates on the virtues—cardinal (prudence, justice, temperance, fortitude) and theological (faith, hope, charity)—as necessary for human flourishing.
Part II-II: Theological Virtues and Sin
Aquinas develops a detailed analysis of the theological virtues and their role in orienting humans toward God. He discusses the nature of sin, its types (original and actual), and its effects on the soul. Aquinas examines repentance, grace, and justification as means of restoration.
Part III: The Person and Work of Christ
This part covers Christology—the study of Jesus Christ as both fully divine and fully human. Aquinas explains the incarnation, the union of divine and human natures, and the necessity of Christ’s atoning sacrifice. He also explores the sacraments as channels of grace instituted by Christ.
Supplement: Eschatology
In the supplementary section, Aquinas addresses last things: death, judgment, heaven, hell, and purgatory. He discusses the resurrection of the body and the final union of the soul with God, concluding his comprehensive treatment of Christian doctrine.