FROM PAPAL ENCYCLICAL LEO XIII, SATIS COGNITUM Every Revealed Truth, without Exception, Must be Accepted
http://www.vatican.va/holy_father/leo_xiii/encyclicals/documents/hf_l-xiii_enc_29061896_satis-cognitum_en.html
9. The Church, founded on these principles and mindful of her office, has done
nothing with greater zeal and endeavour than she has displayed in guarding 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 real 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, whoever 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., 3, 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" (13). But, in what has Christ placed the primary
principle, and the means of preserving this unity? In that-"He gave some
Apostles-and other some pastors and doctors, for the perfecting of the saints,
for the work of the ministry, for the edifying of the body of Christ" (11-12).
Unicorn in the Sanctuary, Randy England, Tan Books, Virginia, 1991
