Solemn Definitions by the Church:
"The universal Church of the faithful is one outside of which none is saved."
-Pope Innocent III, ex cathedra, Fourth Lateran Council (1215 AD)
"We declare, say , define, and pronounce that it is absolutely necessary for the salvation of every human creature to be subject to the Roman Pontiff."
-Pope Boniface VIII, Unam Sanctam (1302 AD)
"The Most Holy Roman Church firmly believes, professes and preaches that none of those existing outside the Catholic Church, not only pagans, also Jews, heretics, and schismatics can ever be partakers of eternal life, but that they are to go into the eternal fire 'which was prepared for the devil and his angels' (Mt. 25:41) unless before death they are joined with Her... No one, let his almsgiving be as great as it may, no one, even if he pour out his blood for the Name of Christ can be saved unless they abide within the bosom and unity of the Catholic Church."
-Pope Eugene IV, ex cathedra, Council of Florence, Cantate Domino (1441 AD)
Further teaching of Popes:
"The holy universal Church teaches that it is not possible to worship God truly except in Her and asserts that all who are outside of Her will not be saved."
-Pope Saint Gregory the Great (590-604)
"By heart we believe and by mouth confess the one Church, not of heretics but the Holy Roman, Catholic, and Apostolic Church outside which we believe that no one is saved."
-Pope Innocent III, Eius exemplo, 18 December 1208 (DS 423)
“Certainly many remarkable authors, adherents of the true philosophy, have taken pains to attack and crush this strange view. But the matter is so self-evident that it is superfluous to give additional arguments. It is impossible for the most true God, who is Truth Itself, the best, the wisest Provider, and the Rewarder of good men, to approve all sects who profess false teachings which are often inconsistent with one another and contradictory, and to confer eternal rewards on their members. For we have a surer word of the prophet, and in writing to you We speak wisdom among the perfect; not the wisdom of this world but the wisdom of God in a mystery. By it we are taught, and by divine faith we hold one Lord, one faith, one baptism, and that no other name under heaven is given to men except the name of Jesus Christ of Nazareth in which we must be saved. This is why we profess that there is no salvation outside the Church.”
-Pope Leo XII, Ubi Primum, #14, 5 May 1824
"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.]"
-Blessed Pope Pius IX, Singulari Quidem, #4, 17 March 1856
"Some say they are not bound by the doctrine which teaches that the Mystical Body of Christ and the Roman Catholic Church are one and the same thing. Some reduce to a meaningless formula the necessity of belonging to the true Church in order to gain eternal salvation. Others finally belittle the reasonable character of the credibility of Christian Faith. These and like errors, it is clear, have crept in among certain of our sons who are deceived by imprudent zeal for souls or by false science."
-Venerable Pope Pius XII, Humani Generis, #27, 12 August 1950
And further teaching from the Saints, as well as Fathers and Doctors of the Church:
"The Church is the entrance to life; all others are thieves and robbers. On this account we are bound to avoid them . . . . We hear it declared of the unbelieving and the blinded of this world that they shall not inherit the world of life which is to come . . . . Resist them in defense of the only true and life giving faith, which the Church has received from the Apostles and imparted to her sons."
St. Irenaeus (130-202), Bishop and Martyr
"No man can find salvation except in the Catholic Church. Outside the Catholic Church one can have everything except salvation. One can have honor, one can have sacraments, one can sing alleluia, one can answer amen, one can have faith in the Name of the Father and the Son and of the Holy Ghost, and preach it too, but never can one find salvation except in the Catholic Church."
-St. Augustine (354-430), Bishop and Doctor of the Church
"Most firmly hold and never doubt that not only pagans, but also Jews, all heretics, and all schismatics who finish this life outside of the Catholic Church, will go into eternal fire prepared for the devil and his angels."
-St. Fulgentius (468-533), Bishop
"All who have not believed that Jesus Christ was really the Son of God are doomed. Also, all who see the Sacrament of the Body of Christ and do not believe it is really the most holy Body and Blood of the Lord . . . these also are doomed!"
-Saint Francis of Assisi (1182-1226)
"There is no entering into salvation outside the Catholic Church, just as in the time of the Flood there was not salvation outside the Ark, which denotes the Church."
-Saint Thomas Aquinas (1226-1274), the Angelic Doctor
"There is no salvation outside the Catholic Church. Anyone who resists this truth perishes."
-Saint Louis Marie de Montfort (1673-1716)
"Outside the Church there is no salvation...therefore in the symbol (Apostles Creed) we join together the Church with the remission of sins: 'I believe in the Holy Catholic Church, the communion of saints, the forgiveness of sins"...For this reason the Church is compared to the Ark of Noah, because just as during the deluge, everyone perished who was not in the ark, so now those perish who are not in the Church."
-Saint Robert Bellarmine (1542-1621), Bishop and Doctor of the Church
"All the misfortunes of unbelievers spring from too great an attachment to the things of life. This sickness of heart weakens and darkens the understanding, and leads to eternal ruin. If they would try to heal their hearts by purging them of their vices, they would soon receive light, which would show them the necessity of joining the Catholic Church, where alone is salvation. We should constantly thank the Lord for having granted us the gift of the true Faith, by associating us with the children of the Holy Catholic Church ... How many are the infidels, heretics, and schismatics who do not enjoy the happiness of the true Faith! Earth is full of them and they are all lost!"
-Saint Alphonsus Maria de Liguori (1696-1787), Bishop and Doctor of the Church