4 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." 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"? 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; and these men, so far from being quite few and scattered, have increased to the dimensions of an entire class, and have grouped themselves into widely spread societies, most of which are directed by non-Catholics, although they are imbued with varying doctrines concerning the things of faith. 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.
Source: Mortalium Animos (Vatican.va)