This is a dogma of the One, Holy, Catholic and Apostolic Church. A dogma is a truth appertaining to faith or morals, revealed by God, transmitted from the Apostles in the Scriptures or by Sacred Tradition and proposed by the Church for the acceptance of the faithful. A dogma cannot be altered or reinterpreted as a dogma is the interpretation as infallibly declared by Pope Piux IX.
Pope Pius IX, First Vatican Council, Session 3, Chapter 2 on Revelation, 1870, Ex Cathedra: "Hence, also, that understanding of its sacred dogmas must be perpetually retained, which Holy Mother Church has once declared; and there must never be a recession from that meaning under the specious name of a deeper understanding."
Pope Pius IX, First Vatican Council, Session 3, Canon 3 on Faith and Reason, 1870, Ex Cathedra: “If anyone says that it is possible that at some time, given the advancement of knowledge, a sense may be assigned to the dogmas propounded by the church which is different from that which the Church has understood and understands: let him be anathema.”

Now that you know a dogma is infallible and unchangeable because it comes directly from God in Heaven. The following dogmas should be as clear as day for you to understand just how important it is to hold the Catholic Faith whole and inviolate.
Pope Eugene IV, Council of Florence, Cantate Domino, 1441, Ex Cathedra: "The most Holy Roman Church firmly believes, professes and preaches that none of those existing outside the Catholic Church, not only pagans, but also Jews and heretics and schismatics, can have a share in life eternal; but that they will go into the eternal fire which was prepared for the devil and his angels, unless before death they are joined with Her; and that so important is the unity of this ecclesiastical body that only those remaining within this unity can profit by the sacraments of the Church unto salvation, and they alone can receive an eternal recompense for their fasts, their almsgivings, their other works of Christian piety and the duties of a Christian soldier. 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 he remain within the bosom and the unity of the Catholic Church."
Pope Eugene IV, Council of Florence, Cantate Domino, 1442, Ex Cathedra: "The Holy Roman Church founded on the words of our Lord and Savior firmly believes, professes and preaches one true God, almighty, immutable and eternal, Father, Son and Holy Spirit. Therefore It [the Holy Roman Church] condemns, rejects, anathematizes and declares to be outside the Body of Christ, which is the Church, whoever holds opposing or contrary views."
Pope Innocent III, Fourth Lateran Council, 1215, Ex Cathedra: "There is indeed one universal Church of the faithful outside of which no one at all is saved..."
Pope Gregory XVI, Mirari Vos (# 13), Aug. 15, 1832, Ex Cathedra: “With the admonition of the apostle that ‘there is one God, one faith, one baptism’ (Eph. 4:5) may those fear who contrive the notion that the safe harbor of salvation is open to persons of any religion whatever. They should consider the testimony of Christ Himself that ‘those who are not with Christ are against Him,’ (Lk. 11:23) and that they disperse unhappily who do not gather with Him. Therefore, ‘without a doubt, they will perish forever, unless they hold the Catholic faith whole and inviolate”(Athanasian Creed)

This is just a handful of Papal pronouncements that have repeated this dogma over the ages. Fake Christians, i.e. protestants and the eastern "orthodox", do not understand Papal infallibility because they are heretics, schismatics and idolaters. They are simply not of the true faith so God allows them to fall deeper into heresy because of the rejection of His Church.
Pope Pius IX, Vatican Council I, 1870, Session 4, Chap. 4: “…the Roman Pontiff, when he speaks ex cathedra [from the Chair of Peter], that is, when carrying out the duty of the pastor and teacher of all Christians in accord with his supreme apostolic authority he explains a doctrine of faith or morals to be held by the universal Church, through the divine assistance promised him in blessed Peter, operates with that infallibility with which the divine Redeemer wished that His Church be instructed in defining doctrine on faith and morals; and so such definitions of the Roman Pontiff from himself, but not from the consensus of the Church, are unalterable.”

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