Image of the Emblem of the Holy Inqusition in New Spain (Mexico)
can be nothing more dangerous than those heretics who admit nearly the whole cycle of doctrine, and yet by one word, as with a drop of poison, infect the real and simple faith taught by our Lord and handed down by Apostolic tradition."
(Leo XIII, quoting, Auctor Tract, de Fide Orthodoxa contra Arianos)
ENCYCLICAL LETTER OF POPE LEO XIII,
ON THE UNITY OF THE CHURCH, JUNE, 1896
"The Church ... has in nothing shown greater zeal and endeavor than
in her efforts to guard the integrity of the faith. Hence she regarded as rebels and expelled from the ranks of her children all who held beliefs on any point of doctrine different from her own. The Arians,
the Montanists, the Novatians, the Quartodecimans, the Eutychians, did not certainly reject all Catholic doctrine; they abandoned only a certain portion of it. Still who does not know that they
were declared heretics and banished from the bosom of the Church? In like manner were condemned all authors of heretical tenets who followed them in subsequent ages. 'There can be nothing more dangerous than those heretics who admit nearly the whole cycle of doctrine, and yet by one word, as with a drop of poison, infect the true and simple faith taught by our Lord and handed down by Apostolic tradition'
(Auctor Tract, de Fide Orthodoxa contra Arianos). 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, him who would recede in the least degree from any point of doctrine proposed by her authoritative Magisterium. Epiphanius, Augustine, Theodoret,
drew up a long list of the heresies of their times. St. Augustine notes that other heresies may spring up, to a single one of which, should any one give his assent, he is by the very fact cut off from Catholic unity. 'No one
who merely disbelieves in all (these heresies) can for that reason regard himself as a Catholic or call himself one. For there may be or may arise some other heresies, which are not set out in this work of ours, and, if any one holds to one single one of these
he is not a Catholic.' (S. Augustinus, De Haeresibus, n. 88). The need of this divinely instituted means for the preservation of unity, about which we speak, is urged by St. Paul in his epistle to the Ephesians. In this he first admonishes
them to preserve with every care concord of minds: 'Solicitous to keep the unity of the Spirit in the bond of peace.' (Eph. iv, j et seq.). And as souls cannot be perfectly united in charity unless minds agree in faith, He
wishes all to hold the same faith: 'One Lord, one faith,' and this so perfectly one as to prevent all danger of error: 'That henceforth we be no more children, tossed to and fro, and carried
about with every wind of doctrine by the wickedness of men, by cunning craftiness, by which they lie in wait to deceive,' (Eph. iv, 14); and this he teaches is to be observed, not for a time only — 'but until we all meet in the unity of faith ...
unto the measure of the age of the fulness of Christ.'"