(1)Interreligious gatherings lead to Relativism; they must be avoided by Catholics.
[S]ince 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.
(2) The danger of Ecumenism; the misinterpretation of Christ's prayer that His disciples might be "one."
[S]ome are more easily deceived by the outward appearance of good when there is question of fostering unity among all Christians. Is it not right, it is often repeated, indeed, even consonant with duty, that all who invoke the name of Christ should abstain from mutual reproaches and at long last be united in mutual charity? Who would dare to say that he loved Christ, unless he worked with all his might to carry out the desires of Him, Who asked His Father that His disciples might be "one."[John xvii, 21] And did not the same Christ will that His disciples should be marked out and distinguished from others by this characteristic, namely that they loved one another: "By this shall all men know that you are my disciples, if you have love one for another"?[John xiii, 35] All Christians, they add, should be as "one": for then they would be much more powerful in driving out the pest of irreligion, which like a serpent daily creeps further and becomes more widely spread, and prepares to rob the Gospel of its strength.
These things and others that class of men who are known as pan-Christians continually repeat and amplify; .... This undertaking is so actively promoted as in many places to win for itself the adhesion of a number of citizens, and it even takes possession of the minds of very many Catholics and allures them with the hope of bringing about such a union as would be agreeable to the desires of Holy Mother Church, who has indeed nothing more at heart than to recall her erring sons and to lead them back to her bosom. But in reality beneath these enticing words and blandishments lies hid a most grave error, by which the foundations of the Catholic faith are completely destroyed.
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 ...
(3) Principles "from which Catholics" must learn regarding the union of "all who call themselves Christians"
(A) There is only one true religion: the religion revealed by God.
A.1 The One True Religion
"God, who at sundry times and in divers manners, spoke in times past to the fathers by...