Heresy is defined by the Catholic Church as "the obstinate denial or obstinate doubt after the reception of baptism of some truth which is to be believed by divine and Catholic faith". The term heresy connotes both the belief in itself, and the attitude towards said belief.

In the canon law of the Catholic Church, excommunication is a form of censure. In the formal sense of the term, excommunication includes being barred not only from the sacraments but also from the fellowship of Christian baptism. The principal and severest censure, excommunication presupposes guilt; and being the most serious penalty that the Catholic Church can inflict, it supposes a grave offense. The excommunicated person is considered by Catholic ecclesiastical authority as an exile from the Church, for a time at least.