Catechism of the Catholic Church §404 — excerpt
How did the sin of Adam become the sin of all his descendants? the whole human race is in Adam "as one body of one man".
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.
Also cites: St. Thomas Aquinas, De malo 4, I · Council of Trent: DS 1511-1512