Catechesis: Sin

Here we must confess that sin comes from one human being, Adam, through whose disobedience all people became sinners and subject to death and the devil. This is called the original sin, or the chief sin. The fruits of this sin are the subsequent evil works, which are forbidden in the Ten Commandments....—SA II:III.1,2

This inherited sin has caused such a deep, evil corruption of nature that reason does not comprehend it; rather, it must be believed on the basis of the revelation in the Scriptures (Ps 51, Rom 5, Ex 33, Gen 3).—SA II:III:3