Pope Pius IX, Singulari Quidem, 4 (17 March 1856)
You see, dearly beloved sons and venerable brothers, how much vigilance is needed to keep the disease of this terrible evil from infecting and killing your flocks. Do not cease to diligently defend your people against these pernicious errors. Saturate them with the doctrine of Catholic truth more accurately each day. Teach them that just as there is only one God, one Christ, one Holy Spirit, so there is also only one truth which is divinely revealed. 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.[Rom 1; Heb 11; Council of Trent, session 6, chap. 8.] 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,[St. Cyprian, epistle 43.] 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.[St. Cyprian,de unitat. Eccl.] 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.[St. Cyprian, epistle 72.]