Answer: The authority a Catholic has to determine that heretics are not members of the Church is Catholic dogma, which teaches us that those who depart from the Faith are considered alien to the Church.
Pope Leo XIII, Satis Cognitum (# 9), June 29, 1896:
“The practice of the Church has always been the same, as is shown by the unanimous teaching of the Fathers, who were wont to hold as outside Catholic communion, AND ALIEN TO THE CHURCH, WHOEVER WOULD RECEDE IN THE LEAST DEGREE FROM ANY POINT OF DOCTRINE PROPOSED BY HER AUTHORITATIVE MAGISTERIUM.”
Moreover, to assert that adhering to this Catholic dogma is to engage in private interpretation, as this objection does, is to assert precisely what Pope St. Pius X condemned in his Syllabus of Errors against the Modernists.
Pope St. Pius X, Lamentabile, The Errors of the Modernists, July 3, 1907, #22:
“The dogmas which the Church professes as revealed are not truths fallen from heaven, but they are a kind of interpretation of religious facts, which the human mind by a laborious effort prepared for itself.”- Condemned
Pope Pius X, Lamentabile, The Errors of the Modernists, July 3, 1907, #54:
“The dogmas, the sacraments, the hierarchy, as far as pertains both to the notion and to the reality, are nothing but interpretations and the evolution of Christian intelligence, which have increased and perfected the little germ latent in the Gospel.”- Condemned
Notice, the idea that dogmas are interpretations is condemned. But that’s exactly what this objection is asserting, whether those who make it will admit it or not. They are saying that to apply the truth of a dogma is “private interpretation.” Further refuting this objection is the fact that, in its Decree on the Sacrament of Order, the Council of Trent solemnly declared that the dogmatic canons are for the use of all the faithful.
Pope Pius IV, Council of Trent, Sess. 13, Chap. 4: “These are the matters which in general it seemed well to the sacred Council to teach to the faithful of Christ regarding the sacrament of order. It has, however, resolved to condemn the contrary in definite and appropriate canons in the following manner, so that all, making use of the rule of faith, with the assistance of Christ, may be able to recognize more easily the Catholic truth in the midst of the darkness of so many errors.”
The word “canon” (in Greek: kanon) means a reed; a straight rod or bar; a measuring stick; something serving to determine, rule, or measure. The Council of Trent is infallibly declaring that its canons are measuring rods for “all” so that they, making use of these rules of Faith, may be able to recognize and defend the truth in the midst of darkness! This very important statement blows away the claim of those who say that using dogmas to prove points is “private interpretation.” Catholic dogma is the authority of all who come to these correct conclusions.
Pope Gregory XVI, Mirari Vos (# 7), Aug. 15, 1832: “… nothing of the things appointed ought to be diminished; nothing changed; nothing added; but they must be preserved both as regards expression and meaning.”
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