Pope Pius IX (1846–1878), Encyclical Quanto conficiamur moerore, August 10, 1863: “And here, beloved Sons and Venerable Brothers, We should mention again and censure a very grave error in which some Catholics are unhappily engaged, who believe that men living in error, and separated from the true faith and from Catholic unity, can attain eternal life. Indeed, this is certainly quite contrary to Catholic teaching. It is known to Us and to you that they who labor in invincible ignorance of our most holy religion and who, zealously keeping the natural law and its precepts engraved in the hearts of all by God, and being ready to obey God, live an honest and upright life, can, by the operating power of divine light and grace, attain eternal life, since God who clearly beholds, searches, and knows the minds, souls, thoughts, and habits of all men, because of His great goodness and mercy, will by no means suffer anyone to be punished with eternal torment who has not the guilt of deliberate sin. But, the Catholic dogma that no one can be saved outside the Catholic Church is well-known; and also that those who are obstinate toward the authority and definitions of the same Church, and who persistently separate themselves from the unity of the Church, and from the Roman Pontiff, the successor of Peter, to whom ‘the guardianship of the vine has been entrusted by the Savior,’ (Council of Chalcedon, Letter to Pope Leo I) cannot obtain eternal salvation. The words of Christ are clear enough: ‘And if he will not hear the Church, let him be to thee as the heathen and publican’ (Matthew 18:17); ‘He that heareth you, heareth Me; and he that dispeth you, despiseth Me; and he that dispiseth Me, despiseth Him that sent Me’ (Luke 10:16); ‘He that believeth not shall be condemned’ (Mark 16:16); ‘He that doth not believe, is already judged’ (John 3:18); ‘He that is not with Me, is against Me; and he that gathereth not with Me, scattereth’ (Luke 11:23). The Apostle Paul says that such persons are ‘perverted and self-condemned’ (Titus 3:11); the Prince of the Apostles calls the ‘false prophets… who shall bring in sects of perdition, and deny the Lord who bought them: bringing upon themselves swift destruction’ (2 Peter 2:1).”[8]
Pope Pius IX The Syllabus of Errors, attached to Encyclical Quanta Cura, 1864: [The following are prescribed errors:] "16. Men can, in the cult of any religion, find the way of eternal salvation and attain eternal salvation. - Encyclical Qui pluribus, November 9, 1846.
“17. One ought to at least have good hope for the eternal salvation of all those who in no way dwell in the true Church of Christ. - Encyclical Quanto conficiamur moerore, August 10, 1863, etc.”
Pope Leo XIII (1878–1903), Encyclical Annum Ingressi Sumus: “This is our last lesson to you; receive it, engrave it in your minds, all of you: by God’s commandment salvation is to be found nowhere but in the Church.”
idem, Encyclical Sapientiae Christianae: “He scatters and gathers not who gathers not with the Church and with Jesus Christ, and all who fight not jointly with Him and with the Church are in very truth contending against God.”
Pope St. Pius X (1903–1914), Encyclical Jucunda Sane: “It is our duty to recall to everyone great and small, as the Holy Pontiff Gregory did in ages past, the absolute necessity which is ours, to have recourse to this Church to effect our eternal salvation.”
Pope Benedict XV (1914–1922), Encyclical Ad Beatissimi Apostolorum: “Such is the nature of the Catholic faith that it does not admit of more or less, but must be held as a whole, or as a whole rejected: This is the Catholic faith, which unless a man believe faithfully and firmly, he cannot be saved.”
Pope Pius XI (1922–1939), Encyclical Mortalium Animos: “The Catholic Church alone is keeping the true worship. This is the font of truth, this is the house of faith, this is the temple of God; if any man enter not here, or if any man go forth from it, he is a stranger to the hope of life and salvation… Furthermore, in this one Church of Christ, no man can be or remain who does not accept, recognize and obey the authority and supremacy of Peter and his legitimate successors.”
Pope Pius XII (1939–1958), Encyclical Humani Generis, August 12, 1950: “Some reduce to a meaningless formula the necessity of belonging to the true Church in order to gain eternal salvation.”
Pope Pius XII (1939–1958), Allocution to the Gregorian University (17 October 1953): “By divine mandate the interpreter and guardian of the Scriptures, and the depository of Sacred Tradition living within her, the Church alone is the entrance to salvation: She alone, by herself, and under the protection and guidance of the Holy Spirit, is the source of truth.”