“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 hears you, hears Me; and he that despises you, despises Me; and he that despises Me, despises Him that sent Me’ (Luke 10:16); ‘He that believeth not shall be condemned’ (Mark 16:16); ‘He that does not believe, is already judged’ (John 3:18); ‘He that is not with Me, is against Me; and he that gathers not with Me, scatters’ (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).”
[Encyclical of Pope Pius IX (1846–1878), On Promotion of False Doctrines, Quanto conficiamur moerore, August 10, 1863].