"No one can deny or doubt that this Jesus Christ, in order to apply the fruits of His Redemption to all generations of man, has built here on earth, upon Peter, His one, holy, Catholic and apostolic Church; and that He conferred upon her the power necessary to preserve whole and inviolate the deposit of Faith, and to transmit this same Faith to all peoples, tribes and nations, so that all men be united in her Mystcal Body through Baptism. Wherefore, this Church, which constitutes His Mystical Body, will persist and prosper in her stable and immutable nature until the end of time
Whoever carefully considers and studies the situation of the various religious communities, divided amongst themselves and separated from the Catholic Church will be easily convinced that none of these associations whether considered individually or taken as a whole can in any way be seen as that One Catholic Church that Christ the Lord built and willed to exist. Neither can they in any way be considered members or part of this same Church, as long as they remain visibly separated from Catholic unity. It follows that such communities, lacking the living authority established by God to teach men especially in Morals and matters of Faith and customs, directing and governing them in all that concerns eternal salvation thus mutate in their doctrines and are constantly changing and instable.
For this reason, let all those who do not possess the communion and the truth of the Catholic Church take advantage of this Council, in which she offers a further demonstration of her profound unity and impregnable vital force; and responding to the demands of their hearts, let them strive to leave this state that does not guarantee for them the security of salvation."
(Pius IX, Apostolic
Letter Iam vos
of September 13, 1868 - DS 2997-2999 )