§1. A person who has publicly rejected the Catholic faith, has defected from ecclesiastical communion, or has been punished by an imposed or declared excommunication cannot be received validly into public associations.
§2. Those enrolled legitimately who fall into the situation mentioned in §1, after being warned, are to be dismissed from the association, with due regard for its statutes and without prejudice to the right of recourse to the ecclesiastical authority mentioned in can. 312, §1.
Source: Code of Canon Law (Vatican.va)