Catechism of the Catholic Church §2110 — excerpt
The first commandment forbids honoring gods other than the one Lord who has revealed himself to his people. It proscribes superstition and irreligion. Superstition in some sense represents a perverse excess of religion; irreligion is the vice contrary by defect to the virtue of religion.
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.