Canon 1325 (1917 CIC) gives the classic definition of the word “heretic”, taken from St Thomas:
Canonists are agreed that the pertinacity in question consists in knowing that the doctrine one denies (or doubts) is taught by the Church as revealed. No other condition, such as authoritative judgment is required to make someone a heretic.
St. Robert Bellarmine’s doctrine on the membership of the Church is the basis for the presentation in the encyclical Mystici Corporis. There, four requirements for membership are given:
(1)those who are baptized,
(2) who profess the Faith integrally,
(3) who submit to the lawful authority of the Pope and hierarchy in communion with him, and
(4) who have not been excluded from the Church by excommunication.
Thus, heretics, schismatics, infidels, and excommunicates are excluded from the Church, even though they are baptized.
Heretics and excommunicates are two different categories. In the case of the former (and schismatics as well), they are excluded by their own actions; in the case of excommunicates, they are excluded by the Church’s judgment, in punishment of crimes committed.
Heresy is a sin. When it is externally manifest, it is a crime that is punished by the Church. A heretic is ipso facto excommunicated.
SFD