Catechism of the Catholic Church
2087 Our moral life has its source in faith in God who reveals his love to us. St. Paul speaks of the "obedience of faith" 9 as our first obligation. He shows that "ignorance of God" is the principle and explanation of all moral deviations. 10 Our duty toward God is to believe in him and to bear witness to him.
2088 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. Involuntary doubt refers to hesitation in believing, difficulty in overcoming objections connected with the faith, or also anxiety aroused by its obscurity. If deliberately cultivated doubt can lead to spiritual blindness.
2089 Incredulity is the neglect of revealed truth or the willful refusal to assent to it. "Heresy is the obstinate post-baptismal denial of some truth which must be believed with divine and catholic faith, or it is likewise an obstinate doubt concerning the same; apostasy is the total repudiation of the Christian faith; schism is the refusal of submission to the Roman Pontiff or of communion with the members of the Church subject to him." 11
English Translation of the Cathechism of the Catholic Church for the United States of America © 1997, United States Catholic Conference, Inc.