Pius XI, in Mortalium Animos (January 6, 1928), wrote the following in paragraph 9:
. . . For this reason it is that
all who are truly Christ's believe, for example, the Conception of the Mother
of God without stain of original sin with the same faith as they
believe the mystery of the August Trinity, and the Incarnation of our Lord just
as they do the infallible teaching authority of the Roman Pontiff, according to
the sense in which it was defined by the Ecumenical Council of the Vatican.
Are these truths not equally certain, or not equally to be believed,
because the Church has solemnly sanctioned and defined them, some
in one age and some in another, even in those times immediately before our own?
Has not God revealed them all? For the teaching authority of the Church, which
in the divine wisdom was constituted on earth in order that revealed doctrines
might remain intact for ever, and that they might be brought with ease and
security to the knowledge of men, and which is daily exercised through the
Roman Pontiff and the Bishops who are in communion with him, has also the
office of defining, when it sees fit, any truth with solemn rites and decrees,
whenever this is necessary either to oppose the errors or the attacks of
heretics, or more clearly and in greater detail to stamp the minds of the
faithful with the articles of sacred doctrine which have been explained. But in
the use of this extraordinary teaching authority no newly invented matter is
brought in, nor is anything new added to the number of those truths which are
at least implicitly contained in the deposit of Revelation, divinely handed
down to the Church: only those which are made clear which perhaps may still
seem obscure to some, or that which some have previously called into question
is declared to be of faith.
In
other words, the Immaculate Conception of Mary (defined 1854) and Papal
Infallibility (defined 1870) are to be believed with the same faith as the
Incarnation of Jesus and other doctrines relating to Christology, Pneumatology,
etc.,, and they are “equally certain” as each other as Rome has
defined them infallibly.
