This dogma has been affirmed many times over by the Churches Magesterium. It has been affirmed by Pope Innocent III (DS 423), The IV Lateran Council (DS 430), Pope Boniface VIII (DS 468), The Council of Florence (DS 714), Pius IX (DS 1647), Pope Clement VI (DS 5706), The Council of Trent (DS 861) etc.

We shall lists some of the Popes and Saints of the Church and what they taught on this Catholic Dogma:

Pope St. Clement I, A.D. 88-97: "Heretical teachers pervert Scripture and try to get into Heaven with a false key, for they have formed their human assemblies later than the Catholic Church. From this previously-existing and most true Church, it is very clear that these later heresies, and others which have come into being since then, are counterfeit and novel inventions." (Epistle to the Corinthians)

Saint Ignatius of Antioch: "Do not deceive yourselves, he who adheres to the author of a schism will not possess the kingdom of God." [Epistle to the Philadelphians, 3 (CH 158)].

Saint Cyprianus: "Whosoever is separated from the Church is united to an adulteress.  He has cut himself off from the promises of the Church, and he who leaves the Church of Christ cannot arrive at the rewards of Christ (...)  He who observes not this unity observes not the law of God, holds not the faith of the Father and the Son, clings not to life and salvation." [De Cath. Eccl. Unitate, n 6 (CH 555)].

Council of Nicea (first ecumenical council, A.D. 325): "Let the patriarch consider what things are done by the archbishops and bishops in their provinces; and if he shall find anything done by them otherwise than it should be, let him change it and order it, as seemeth to him fit; for he is the father of all, and they are his sons. And although the Archbishop be among the bishops as an elder brother, who hath the care of his brethren, and to whom they owe obedience because he is over them; yet the patriarch is to all those who are under his power, just as he who holds the seat of Rome is the head and prince of all patriarchs; inasmuch as he is first, as was Peter, to whom power is given over all Christian princes, and over all their peoples, as he who is the Vicar of Christ our Lord over all peoples and over the whole Christian Church, and whoever shall contradict this, is excommunicated by the synod." (Arabic Canons, Canon XXXIX)

The Synod of Laodicea, A.D. 343-381: "Canon XXXIV. No Christian shall forsake the martyrs of Christ, and turn to false martyrs, that is, to those of the heretics, or those who formerly were heretics; for they are aliens from God. Let those who go after them be anathema."

"Ancient Epitome of Canon XXXIV. Whosoever honours an heretical pseudo-martyr, let him be anathema."

Saint Augustine and the Council of Cirta (412 A.D.): "He who is separated from the body of the Catholic Church, however laudable his conduct may otherwise seem, will never enjoy eternal life, and the anger of God remains on him by reason of the crime of which he is guilty in living separated from Christ." [Epist. 141 (CH 158)].

Saint Gregory the Great: "The holy universal Church teaches that God cannot be truly adored except within its fold; she affirms that all those who are separated from her will not be saved." [Moral. in Job. XIV,5 (CH 158)].

Saint Jerome (died A.D. 420): "As I follow no leader save Christ, so I communicate with none but your blessedness, that is, with the Chair of Peter. For this, I know, is the rock on which the Church is built. ...This is the ark of Noah, and he who is not found in it shall perish when the flood prevails. ...And as for heretics, I have never spared them; on the contrary, I have seen to it in every possible way that the Church's enemies are also my enemies." (Manual of Patrology and History of Theology)

Saint Augustine (died A.D. 430): "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 the sacraments, one can sing alleluia, one can answer amen, one can have faith in the name of the Father and of the Son and of the Holy Ghost, and preach it too, but never can one find salvation except in the Catholic Church." (Sermo ad Caesariensis Ecclesia plebem)

Saint John Chrysostom, Doctor, (died A.D. 407): "We know that salvation belongs to the Church alone, and that no one can partake of Christ nor be saved outside the Catholic Church and the Catholic Faith." (De Capto Eutropia)

Saint Fulgentius (died A.D. 533): "Most firmly hold and never doubt that not only pagans, but also all Jews, all heretics, and all schismatics who finish this life outside of the Catholic Church, will go into the eternal fire prepared for the devil and his angels." (Enchriridion Patristicum)

Pope Pelagius II (A.D. 578-590): "Consider the fact that whoever has not been in the peace and unity of the Church cannot have the Lord. Although given over to flames and fires, they burn, or, thrown to wild beasts, they lay down their lives, there will not be for them that crown of faith but the punishment of faithlessness. Such a one can be slain, he cannot be crowned. If slain outside the Church, he cannot attain the rewards of the Church." (Denzinger 246-247)

Pope Saint Gregory the Great (A.D. 590-604): "Now the holy Church universal proclaims that God cannot be truly worshipped saving within herself, asserting that all they that are without her shall never be saved." (Moralia)

Saint Bede the Venerable O.S.B., Doctor, (died A.D. 735): "He who will not willingly and humbly enter the gate of the Church will certainly be damned and enter the gate of hell whether he wants to or not." (Sermon 16) "Without this confession, without this faith, no one can enter the kingdom of God." (Sermon 16)

Saint Peter Mavimenus (died A.D. 743): "Whoever does not embrace the Catholic Christian religion will be damned, as was your false prophet Mohammed." (Roman Martyrology, February 20th) [Upon this profession of the faith, the infidel murdered him.]

Pope Sylvester II, A.D. 999-1003: "I profess that outside the Catholic Church, no one is saved." (Profession of Faith made as Archbishop of Rheims, June 991; Letters of Gerbert, NY: Columbia University Press.) [This is the man that introduced Arabic numerals (the ones we use) into the West.]

Pope Saint Leo IX, A.D. 1049-1054): [regarding the eastern so-called "Orthodox" schismatics]: "If you live not in the body which is Christ, you are none of His. Whose, then, are you? You have been cut off and will wither, and like the branch pruned from the vine, you will burn in the fire - an end which may God's goodness keep far from you."

Innocent III and the Fourth Ecumenical Council of the Lateran (1215 A.D.): "There is only one universal Church of the faithful, outside of which no one can be saved." [Cap. I; De fide cath.; DS 802 (CH 159)].

Saint Thomas Aquinas (died A.D. 1274): "There is no enterning into salvation outside the Church, just as in the time of the deluge there was none outside the ark, which denotes the Church." (Summa Theologiae)

Pope Eugene IV: "Whoever wishes to be saved needs, above everything else, to hold the Catholic faith. Unless each one preserves this faith whole and inviolate, he will perish in eternity without a doubt." - Exultate Deo," DZ 695

Pope Adrian II "The first requirement of salvation is to keep to the standard of the true faith." Actio I," DZ 171, n.1

Pope Gregory XVI - "He who is separated from the body of the Catholic Church, however praiseworthy his conduct may otherwise seem, will not be saved." "Perlatum Ad Nos," PTC:186; "Summo Jugiter," PTC:158

Pope Leo XIII, Satis Cognitum: “The Church...regarded as rebels and expelled from the ranks of her children all who held beliefs on any point of doctrine different from her own...The practice of the Church has always been the same, as is shown by the unanimous teaching of the Fathers, who were wont to hold as outside Catholic communion, and alien to the Church, whoever would recede in the least degree from any point of doctrine proposed by her authoritative Magisterium...Whosoever is separated from the Church is united to an adultress. He has cut himself off from the promises of the Church, and he who leaves the Church of Christ cannot arrive at the rewards of Christ...He who observes not this unity observes not the law of God, holds not the faith of the Father and the Son, clings not to life and salvation.”

Pope Pius IV "I promise, vow, and swear that, with God's help, I shall most constantly hold and profess this true Catholic faith, outside which no one can be saved." : from the Bull "Injunctum Nobis," DZ:1000

Pope Pius VIII "Remember this firm dogma of our religion: that outside the true Catholic faith no one can be saved." - RECOLLECTIONS OF THE LAST FOUR POPES, Cardinal Nicholas Wiseman, London: 1858

Pope Pius IX "See to it that the faithful have fixed firmly in their minds this dogma of our most holy religion: the absolute necessity of the Catholic faith for attaining salvation." - Nostis et Nobiscum," December 8, 1849

Pope St. Pius X "Where is the road which leads us to Jesus Christ? It is the Church. It is our duty to recall to everyone, great and small, the absolute necessity we are under to have recourse to this Church in order to work out our eternal salvation." - "Supremi Apostolatus," PTC:654; "Jucunda Sane," PTC:668

Pope Pius XI "If any man does not enter the Church, or if any man departs from it, he is far from the hope of life and salvation." - Mortalium Animos," PTC:873

Pope Pius XII  "No one can depart from the teaching of Catholic truth without loss of faith and salvation." - Pius XII: "Ad Apostolorum Principis," PTC:1536

Council of Trent - "Constantly hold and profess this true Catholic faith, without which no one can be saved. Tridentine Profession of Faith, DZ:1000

Vatican I "This true Catholic faith, outside which no one can be saved, which I now freely profess and truly hold, I do promise and swear that I will most constantly keep and confess whole and inviolate with the help of God until the last breath of my life, and that I will take great care that it be held, taught, and preached by my inferiors and by those who are placed under my charge." - Papal Oath

Saint Peter Canisius (died A.D. 1597): "Outside of this communion - as outside the ark on Noah - there is absolutely no salvation for mortals: not for Jews or pagans who never recieved the faith of the Church, nor for heretics who, having recieved it, corrupted it; neither for the excommunicated or those who for any other serious cause deserve to be put away and separated from the body of the Church like pernicious members...for the rule of Cyprian and Augustine is certain: he will not have God for his Father who would not have the Church for his mother." (Cathechismi Latini et Germanici)

Saint Robert Bellarmine (died A.D. 1621): "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 with 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." (De Sacramento Baptismi)

St. Francis of Assisi "And all of us humbly entreat and beseech everyone, all nations and all men in all the earth who are, and who shall be, that we may all of us persevere in the true faith: for otherwise no one can be saved."

St. Louis Marie de Montfort "There is no salvation outside the Catholic Church. Anyone who resists this truth perishes." -

St. Alphonsus Maria Liguori "We must believe that the Roman Catholic Church is the only true Church; hence, they who are out of our Church, or they who are separated from it, cannot be saved." :

"The same teaching is expressed in the professions of faith which have been proposed of by the Apostolic See; in the one which all the Latin Churches use (DS 1870); as also in the others, one which is received by the Greeks (cf. Gregory XIII: Prof. XXX; DS 1985. ), and the other by all other Eastern Catholics" [Benedict XIV: Const. Nuper ad Nos; DS 2540 Cf. Also Pope Gregory XVI: Encyclical Summo jugiter, May 27, 1832 to the Bishops of Bavaria (CH 159)].