"Here, too, our beloved sons and venerable brothers, it is again necessary to mention and censure a very grave error entrapping some Catholics who believe that it is possible to arrive at eternal salvation although living in error and alienated from the true faith and Catholic unity. Such belief is certainly opposed to Catholic teaching. There are, of course, those who are struggling with invincible ignorance about our most holy religion. Sincerely observing the natural law and its precepts inscribed by God on all hearts and ready to obey God, they live honest lives and are able to attain eternal life by the efficacious virtue of divine light and grace. Because God knows, searches and clearly understands the minds, hearts, thoughts, and nature of all, his supreme kindness and clemency do not permit anyone at all who is not guilty of deliberate sin to suffer eternal punishments.

8. Also well known is the Catholic teaching that no one can be saved outside the Catholic Church. Eternal salvation cannot be obtained by those who oppose the authority and statements of the same Church and are stubbornly separated from the unity of the Church and also from the successor of Peter, the Roman Pontiff, to whom "the custody of the vineyard has been committed by the Savior."[4] The words of Christ are clear enough: "If he refuses to listen even to the Church, let him be to you a Gentile and a tax collector;"[5] "He who hears you hears me, and he who rejects you, rejects me, and he who rejects me, rejects him who sent me;"[6] "He who does not believe will be condemned;"[7] "He who does not believe is already condemned;"[8] "He who is not with me is against me, and he who does not gather with me scatters."[9] The Apostle Paul says that such persons are "perverted and self-condemned;"[10] the Prince of the Apostles calls them "false teachers . . . who will secretly bring in destructive heresies, even denying the Master. . . bringing upon themselves swift destruction."[11]"

http://www.papalencyclicals.net/Pius09/p9quanto.htm

There is only one divine faith which is the beginning of salvation for mankind and the basis of all justification, the faith by which the just person lives and without which it is impossible to please God and to come to the community of His children.[2] There is only one true, holy, Catholic church, which is the Apostolic Roman Church. There is only one See founded in Peter by the word of the Lord,[3] outside of which we cannot find either true faith or eternal salvation. He who does not have the Church for a mother cannot have God for a father, and whoever abandons the See of Peter on which the Church is established trusts falsely that he is in the Church.[4] Thus, there can be no greater crime, no more hideous stain than to stand up against Christ, than to divide the Church engendered and purchased by His blood, than to forget evangelical love and to combat with the furor of hostile discord the harmony of the people of God.[5]

http://www.papalencyclicals.net/Pius09/p9singul.htm