These are some quotes from some recent popes to add to the conversation.
“Outside this Body, the Holy Spirit gives life to NO ONE; those outside the Church do not possess the Holy Spirit. The Catholic Church alone is the Body of Christ, and if a man be separated from the Body of Christ he is not one of His members nor is he fed by His Spirit.” Pope Paul VI
“We must mention another fruitful cause of evil by which the Church is afflicted at present, namely: Indifferentism, that vicious manner of thinking which holds that eternal salvation can be obtained by the profession of any faith, provided that a man’s morals are good and decent. Seriously consider the testimony of the Savior that some are against Christ because they are not with Christ, that they scatter who do not gather with Him, and therefore without doubt they will perish in eternity unless they hold to the Catholic faith and observe it WHOLE and INVIOLATE.” Pope Gregory XVI
“It is an error in a matter of divine truth to imagine the Church as invisible, by which many Christian communities, although they differ from each other in their faith, are united by a bond that is invisible to the senses.” Pope Pius XII
“Faith in Christ cannot be maintained pure and unalloyed when it is not protected and supported by faith in the Church. Faith in Christ and faith in the Church stand together. If any man does not enter the Church, or if any man departs from her, he is far from the hope of life and salvation.” Pope Pius XI
“The Catholic Church alone is the Body of Christ, of which He is Head and Savior. We must always remember the unity of the Mystical Body outside which there is no salvation; for their is no entering into salvation outside the Church. Truth, grace, the Sacraments: all the certain norms for our journey to God come from the Church. The Catholic Church is the extension of Jesus Christ in time and space. Outside this Body the Holy Spirit does not give life to anyone. Those who are enemies ot unity do not participate in the charity of divine life; those outside the Church do not possess the Holy Spirit. A Christian must fear nothing so much as to be separated from the Body of Christ. If he is separated from Christ’s Body, he is not one of His members; he is not fed by His Spirit.” Pope Paul VI
The ruin of souls is wrought by this single cause: Ignorance of those most sublime truths, so far beyond the natural understanding of the multitutde, which nonetheless must be known by all men in order that they may attain eternal salvation. We positively maintain that the will of man cannot be upright, nor his conduct good, while his intellect is the slave of crass ignorance. This We solemnly affirm: the majority of those who are condemned to eternal punishment fall into this everlasting misfortune through ignorance of the mysteries of the Faith which must necessarily be known and believed by all who belong to the Elect. Pope St. Pius X
“Since it is recognized that it is extremely rare to find men entirely devoid of religious sense, some people entertain the hope that nations, in spite of their differing religious viewpoints, may be brought to unite as brothers in the profession of certain doctrines as a common foundation of the spiritual life. Certainly, such efforts as these cannot receive the approval of Catholics, for they rest on the false opinion which regards any religion whatsoever to be more-or-less praiseworthy and good. Those who hold this opinion are in grave error; they even debase the concept of the true religion and lapse, little by little, into naturalism and atheism.” Pope Pius XI
As Christ is the head of the Church, so is the Holy Ghost her soul. ONLY those are really to be included as members of the Church who have been baptized and who profess the true Faith and who have not unhappily withdrawn from the Body or, for grave reasons, been excluded by legitimate authority. It follows that those who are divided in faith or in government cannot be living in one Body such as this, and cannot be living the life of its one Divine Spirit. Pius XII
“By means of religious Indifferentism, crafty men deceitfully pretend that people can attain eternal salvation in the practice of any religion, as though there could be any fellowship of light with darkness. These men conclude that not only sons of the Church but also others, however estranged they may remain from Catholic unity, are equally on the road to salvation and are able to achieve everlasting life. Words fail Us from utter HORROR in detesting and abhorring this new and terrible insult!” Pope Pius IX