Catechism of the Catholic Church §2088 — excerpt
The first commandment requires us to nourish and protect our faith with prudence and vigilance, and to reject everything that is opposed to it. There are various ways of sinning against faith: Voluntary doubt about the faith disregards or refuses to hold as true what God has revealed and the Church proposes for belief.
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.