In the Encyclical "Mortalium Animos" ("On Religious Unity") Pope Pius XI explicitly condemns what many of us Catholics in the 21st century shockingly believe is a "good thing". The Encyclical can be read in its entirety on the Vatican website.
Here are a few excerpts from the Encyclical, (though I highly recommend reading the entire thing to fully understand where the Pope is coming from.)
"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."
"(H)ere it seems opportune to expound and to refute a certain false opinion, on which this whole question, as well as that complex movement by which non-Catholics seek to bring about the union of the Christian churches depends. For authors who favor this view are accustomed, times almost without number, to bring forward these words of Christ: "That they all may be one.... And there shall be one fold and one shepherd," with this signification however: that Christ Jesus merely expressed a desire and prayer, which still lacks its fulfillment. For they are of the opinion that the unity of faith and government, which is a note of the one true Church of Christ, has hardly up to the present time existed, and does not to-day exist. They consider that this unity may indeed be desired and that it may even be one day attained through the instrumentality of wills directed to a common end, but that meanwhile it can only be regarded as mere ideal. They add that the Church in itself, or of its nature, is divided into sections; that is to say, that it is made up of several churches or distinct communities, which still remain separate, and although having certain articles of doctrine in common, nevertheless disagree concerning the remainder; that these all enjoy the same rights; and that the Church was one and unique from, at the most, the apostolic age until the first Ecumenical Councils. Controversies therefore, they say, and longstanding differences of opinion which keep asunder till the present day the members of the Christian family, must be entirely put aside, and from the remaining doctrines a common form of faith drawn up and proposed for belief, and in the profession of which all may not only know but feel that they are brothers. The manifold churches or communities, if united in some kind of universal federation, would then be in a position to oppose strongly and with success the progress of irreligion. This, Venerable Brethren, is what is commonly said. There are some, indeed, who recognize and affirm that Protestantism, as they call it, has rejected, with a great lack of consideration, certain articles of faith and some external ceremonies, which are, in fact, pleasing and useful, and which the Roman Church still retains. They soon, however, go on to say that that Church also has erred, and corrupted the original religion by adding and proposing for belief certain doctrines which are not only alien to the Gospel, but even repugnant to it. Among the chief of these they number that which concerns the primacy of jurisdiction, which was granted to Peter and to his successors in the See of Rome. Among them there indeed are some, though few, who grant to the Roman Pontiff a primacy of honor or even a certain jurisdiction or power, but this, however, they consider not to arise from the divine law but from the consent of the faithful. Others again, even go so far as to wish the Pontiff Himself to preside over their motley, so to say, assemblies. But, all the same, although many non-Catholics may be found who loudly preach fraternal communion in Christ Jesus, yet you will find none at all to whom it ever occurs to submit to and obey the Vicar of Jesus Christ either in His capacity as a teacher or as a governor. Meanwhile they affirm that they would willingly treat with the Church of Rome, but on equal terms, that is as equals with an equal: but even if they could so act. it does not seem open to doubt that any pact into which they might enter would not compel them to turn from those opinions which are still the reason why they err and stray from the one fold of Christ.
This being so, it is clear that the Apostolic See cannot on any terms take part in their assemblies, nor is it anyway lawful for Catholics either to support or to work for such enterprises; for if they do so they will be giving countenance to a false Christianity, quite alien to the one Church of Christ. Shall We suffer, what would indeed be iniquitous, the truth, and a truth divinely revealed, to be made a subject for compromise? For here there is question of defending revealed truth."
"We do know that from this it is an easy step to the neglect of religion or indifferentism and to modernism, as they call it. Those, who are unhappily infected with these errors, hold that dogmatic truth is not absolute but relative, that is, it agrees with the varying necessities of time and place and with the varying tendencies of the mind, since it is not contained in immutable revelation, but is capable of being accommodated to human life."
"So, Venerable Brethren, it is clear why this Apostolic See has never allowed its subjects to take part in the assemblies of non-Catholics: for the union of Christians can only be promoted by promoting the return to the one true Church of Christ of those who are separated from it, for in the past they have unhappily left it."
We must not think that believing in this age-old Truth (that Catholicism is the only true religion made by Christ Himself for our salvation,) somehow entitles us to hatred of towards other souls who are not in the Fold. Nor does it entitle us to presumption of one's eternal life, for we can only judge the sin, we cannot claim that a person is in Hell for we do not know their last moments on this earth. However, by the examples and lives of the Saints, we can certainly recognize the grave importance that was held, and must again be held, by believing Catholics concerning the seriousness of sin and the great purchase of even one soul. For Christ died the most agonizing death for each soul, individually and personally, that it may choose Him and be counted among the Saints in Heaven. This is why Saints have such great love for souls, especially the most wayward of souls, for those souls are ever more the triumphant victory of Christ if they be won.
Finally, in facing this topic, in light of the great number of Catholics preaching false ecumenism, let us ask ourselves just one question...
If the Faith is simply, "one of the many paths to salvation" rather than the only path, as taught by the Church, why did the Saints and Martyrs in the past 2,000 years cling so vigorously to the Church's teaching on salvation, that they suffered the most unspeakable torments, and oftentimes, horrendous and violent death for the salvation of souls... for the conversion of souls to the Catholic Church?
I am come hither to die for my religion, for that religion which was founded by Christ, established by the Apostles, and propagated through all the ages since, to the present day, by a visible hierarchy: a religion that rests on the testimony of the Scriptures, supported by the authority of the Fathers and Councils, outside which there can be no hopes of salvation.
- St. Henry Morse
There is no entering into salvation outside the Church, just as in the time of the Flood there was no salvation outside the Ark which denotes the Church.
- St. Thomas Aquinas
Anyone who is outside this Church ... is walking a path not to Heaven but to Hell. He is not getting closer to the home of eternal. life; on the contrary, he is hurrying to the torment of eternal death. And this is the case not only if he remains a pagan without Baptism, but even if he continue as a heretic after having been baptized.
- St. Fulgentius
There is no salvation outside the Catholic Church. Anyone who resists this truth perishes.
- St. Louis Marie de Montfort
Peter, in showing that the Church is one and that only those who are in the Church can be saved, said: "In the Ark of Noah certain persons, numbering only eight, were saved by water, which Baptism effects in like manner for you" (1 Peter 3:20). He proves and demonstrates that the solitary Ark of Noah was the figure of the One Church. If, at the time of this Baptism of the world anyone could have been saved without having been in the Ark of Noah, then he who is outside the Church could now be brought to life by Baptism.
- St. Cyprian
The Church gives us to understand that some men can receive even Baptism outside her, but that no one can either receive or possess salvation outside her. Thus, the Baptism of the Church can exist outside the Church, but the gift of a blessed life is not found except within the Church which was founded on a Rock and received the keys of binding and loosing. It is she who keeps and possesses every power of her Spouse and Lord. For the water of the Church is salutary and holy for those who use it well, but outside the Church no one can use it well ... It may be that someone may have Baptism apart, but that it should do him good is impossible ... This man says: "I have Baptism!" Yes, you have it, but Baptism without Charity profits you nothing, because without Charity you are nothing (1 Corinthians 13:2). For outside the Church you would have Baptism to destruction. If you have it inside, Baptism begins to profit you towards salvation.
- St. Augustine
It is impossible to be joined to God except through Jesus Christ; it is impossible to be united to Christ except in and through the Church which is His Mystical Body.
- Pope St. John XXIII
Neither sanctity nor salvation can be found outside the Holy, Catholic, Apostolic, Roman Church.
- Bl. Pope Pius IX
There is but one plain known road ... when you wander from this, you are lost. You must be altogether within the House of God, within the walls of salvation, to be sound and safe from injury. If you wander and walk abroad ever-so-little, if you carelessly thrust hand or foot out of the Ship ... you shall be thrust forth: the door is shut, the ocean roars, you are undone.
- St. Edmund Campion
There can be only one Catholic Church, of which I am a member. This, with the help of God, I will profess till my dying hour. Rest assured, outside the Catholic Church there is no salvation.
- Ven. Francis Bel