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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Matthew 16:18-19, Jesus gives the Keys of the Kingdom of Heaven to Peter, upon this rock, and gave him the power that whatever he binds or loose will be bound or loosed in Heaven. John 21:15-17 Jesus entrusts His entire flock to Saint Peter, He gave Saint Peter authority over his entire flock that he became Pope "Feed my lambs...Tend my sheep...Feed my sheep".

Isaiah 22:20-22 tells us the prime minister had the key of the house of David. The prime minister had successors. Jesus is the King yet when Jesus leaves the world and Ascends into Heaven He gives authority over the Church to Saint Peter and Saint Peter then holds the place of the king. That is why Saint Peter's place in Scripture parallels both the position of prime minister under old testament kings and of king David himself, 1 Chronicles 28:4 and Acts 15:7. In the Davidic monarchy there was not only a king who ruled over all the people but a royal cabinet. Jesus' royal cabinet which parallels the 12 tribes of Israel and the royal cabinet in the Davidic monarchy is the 12 apostles however of all the kings ministers in the Davidic monarchy there was one who stood out with authority amongst the rest, that was the prime minister who was over the kings house. That is why Saint Peter's position throughout the acts of the apostles is that of the Church's leader and is why his role at the council of Jerusalem parallels king David's position. During the council of Jerusalem "And after there had been much debate, Peter stood up and addressed them..." Acts 15:7, Scripture describes and identifies Peter as standing up at the Acts 15 council in Jerusalem just as it describes and identifies king David standing up at the 1 chronicles 28 council in Jerusalem because Peter as the Leader and first Pope of the Church had supreme authority over the new Israel, then Church. Just as king David had supreme authority over old testament Israel. King David in 1 Chronicles 28:4 stands up at the council in Jerusalem and stated "the Lord God of Israel chose me..." the council that concerned how to build the temple of God and said the Lord God chose me, in Acts 15:7 at the new testament council of Jerusalem , the council that concerned how to build the Church of God we read that Peter said this "Brothers, you know that in the early days God chose among you, that by my mouth the gentiles should hear the word of the gospel and believe." the parallel is stunning and clear. Acts 15:12 Scripture teaches that the debate was ongoing prior to Peter's speech but that it ended as soon as Peter's position was articulated and his speech ended. Barnabas, Paul and James proceeded to address the Church after Peter had already defined the core doctrinal truth and principal that would endure throughout the ages, that is gentile converts do not need to be circumcised and all the requirements of the old law.

Saint Peters name is mentioned over 190 times in the New Testament the next closest apostle is John with only 29 mentions, the other 11 apostles combined are only mentioned 130 times, far fewer than Peters. Peter leads the Church in replacing judas in Acts 1, Peter speaks for the Church at Pentecost Acts 2, Peter works the first miraculous healing in post-Ascension Church history in Acts 3, Peter answers for the Church before the council of the high priest in Acts 4, Peter meets out the discipline to Ananias and Sapphira in Acts 5, in acts 10 the first gentile convert is specifically told to find Peter and much more. Every list of the apostles in the New testament has Peters name first and judas' name last, this is true even though the order of the other apostles in between is not always the same. Saint Peter is not only mentioned first but he is directly called first or chief, Matthew 10:2, the Greek word used is Protos (πρῶτος) which means first, chief or principal. Since no other numbers are given in the list and Saint Peter was not the first one to follow Jesus but Saint Andrew was, this statement is not meant to assign a number to Simon Peter it is to indicate that he is the chief, leader or principal of the twelve. Matthew is literally saying "The Chief, Peter". When Saint Peter is mentioned by name the other apostles are frequently described as, those with Peter, Acts 2:37, Acts 5:29, Mark 1:36 and Luke 9:32. Even the angel after the Resurrection does this "But go your way, tell his disciples and Peter that he goes before you into Galilee..." Mark 16:7. Scripture repeatedly singles out Saint Peter and sets him apart from the other apostles because he was the leader, the first Pope.