Catechism of the Catholic Church §2125 — excerpt
Since it rejects or denies the existence of God, atheism is a sin against the virtue of religion. The imputability of this offense can be significantly diminished in virtue of the intentions and the circumstances.
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: GS 19 # 3