5 Admonished, therefore, by the consciousness of Our Apostolic office that We should not permit the flock of the Lord to be cheated by dangerous fallacies, We invoke, Venerable Brethren, your zeal in avoiding this evil; for We are confident that by the writings and words of each one of you the people will more easily get to know and understand those principles and arguments which We are about to set forth, and from which Catholics will learn how they are to think and act when there is question of those undertakings which have for their end the union in one body, whatsoever be the manner, of all who call themselves Christians.
Source: Mortalium Animos (Vatican.va)