Well, according to Canon Law and the practice of the Church, heresy is a formal offense. Heresy, as defined by Canon Law, carries the punishment of excommunication. When someone commits the crime of heresy, it means that person must obstinately deny a truth of faith, a revealed truth which the Church proposes to be believed as a divine truth, or essentially as a dogma of faith.
For example, if someone says that the Son of God, Jesus Christ, is not consubstantial with the Father, such a statement is formal heresy. We proclaim in the dogma and in the symbol of faith, "I believe in Jesus Christ, who is consubstantial with the Father." Another example is if someone says that Our Lady was not conceived immaculately, this, too, is formal heresy. Or if someone denies the Real Presence of Jesus Christ in the Eucharist, that He is not really and substantially present, this is also formal heresy.
When someone publicly and obstinately states such things, he commits the crime of heresy and is punished by excommunication.
If that is not the case, meaning the person denies a truth that is divinely revealed but has not yet been formally proposed by the Church as a dogma of faith, then it is considered material heresy. Material heresy is not punished automatically by excommunication.
Christopher Wendt: So, in the case of formal heresy, is it an automatic excommunication, or does it have to be declared?
Bishop Schneider: It is automatic, but it can also be declared by the Church. Yes, it can also be declared.