[piusv]

A similar object is aimed at by some, in those matters which concern the New Law promulgated by Christ our Lord. For since they hold it for certain that men destitute of all religious sense are very rarely to be found, they seem to have founded on that belief a hope that the nations, although they differ among themselves in certain religious matters, will without much difficulty come to agree as brethren in professing certain doctrines, which form as it were a common basis of the spiritual life. For which reason conventions, meetings and addresses are frequently arranged by these persons, at which a large number of listeners are present, and at which all without distinction are invited to join in the discussion, both infidels of every kind, and Christians, even those who have unhappily fallen away from Christ or who with obstinacy and pertinacity deny His divine nature and mission. Certainly such attempts can nowise be approved by Catholics, founded as they are on that false opinion which considers all religions to be more or less good and praiseworthy, since they all in different ways manifest and signify that sense which is inborn in us all, and by which we are led to God and to the obedient acknowledgment of His rule. Not only are those who hold this opinion in error and deceived, but also in distorting the idea of true religion they reject it, and little by little. turn aside to naturalism and atheism, as it is called; from which it clearly follows that one who supports those who hold these theories and attempt to realize them, is altogether abandoning the divinely revealed religion.

-Mortalium Animos, On Religious Unity, Pope Pius XI, 1928

[vladimir998]

More important and definitive would be: Pope St. Pius X, Letter Our Apostolic Mandate on Society and Interreligious Cooperation (1910). There might be a copy online somewhere.

[Elsie]

Certainly such attempts can nowise be approved by Catholics, founded as they are on that false opinion which considers all religions to be more or less good and praiseworthy, since they all in different ways manifest and signify that sense which is inborn in us all, and by which we are led to God and to the obedient acknowledgment of His rule.

They could sure write sentences back then!

[daniel1212]

Not only are those who hold this opinion in error and deceived, but also in distorting the idea of true religion they reject it, and little by little. turn aside to naturalism and atheism, as it is called; from which it clearly follows that one who supports those who hold these theories and attempt to realize them, is altogether abandoning the divinely revealed religion. Thus we have one of the many interpretations of Rome by a RC, while they censure Prots for doing to their supreme authority, and tell us we need a supreme head to settle matters,