Thomas Aquinas On Original Sin, Part 3.

From Summa Theologica:

Treatise on Habits in Particular, Question 81, Article 3: Whether the sin of the first parent is transmitted, by the way of origin, to all men?

This article is basically concerned with affirming the doctrine of Original Sin. Basically, Thomas relies on the teaching of the "Catholic Faith" (which many today would understand as an appeal to tradition). The one text of Scripture he relies on (appropriately enough) is Romans 5:12- "Therefore as through one man sin entered into the world, and death through sin; and so death passed unto all men, for that all sinned:" (ASV). This verse clearly connects the sin of all men to the sin of Adam.

Thomas draws out the analogy present in Romans 5, showing that all men are sinners being from Adam corporally, whereas all believers have become righteous being from Christ spiritually.