It may seem a bit arrogant, but I think this might be the shortest summary of the historical development of the Roman Catholic Church and the basic reason why there is such a thing as a Protestant.
My purpose in this explanation is not to throw negative light on anyone who claims to be Catholic or anyone who is a member of the Roman Catholic Church. Not all Catholics believe the same things, just like not all Baptists believe the same things, and there are many members of churches (of all sorts) who have no idea what their church actually teaches on a given subject. So, I invite further conversation; I do not purposefully condemn any particular reader or someone you might know.
First, it is complicated, but Catholics have no unique claim on history.
Simply put, the Roman Catholic Church as it is today, in its doctrines and in its administration, did not exist until (at the earliest) the year 1215 AD. The Fourth Lateran Council ratified some of the teachings and most of the organizational forms that are distinctive of and essential to Roman Catholicism today.1 But it was not until the Council of Trent, which met sporadically from 1545 to 1563 that the main doctrines which separate Rome from Protestants were clearly articulated and ratified.2 Therefore, regardless of what my Roman Catholic friends might say, the Roman Church is not the oldest and most united church. It has a complicated past, and it has no unique claim on the Apostles or early Christians.
Second, Catholics and Protestants alike see the need for reform in the late Middle Ages.
Before and during the 1500s, there were many Christians within the Roman Church who were calling for reform. At least as early as the 1300s, with John Wycliffe in England in and Jan Hus in Bohemia (as well as many others), good Roman Catholics were writing and preaching and working for reforms within the Roman Catholic Church. By all observers, including Roman Catholics, Western Christianity had become so abusive and scandalous that something had to change.
Many historians look back and see that the leadership of the Roman Church was unwilling to change, so Catholic priests, local friars, and Church theologians started protesting. The quintessential moment which seems to capture the scene in the early 1500s was that evening of October 31, 1517, when Martin Luther (a German monk, Catholic priest, and promising theologian) nailed his invitation to scholarly debate on the castle-church door in Wittenberg. The nailing of the 95 theses was a historic moment, but there were others like it happening all over Europe.
Zwingli, in Zurich, encouraged his congregation to eat meat during a Roman fasting day. English men and women were sharing copies of Wycliffe’s translation of the Latin Bible, and they were illegally memorizing passages to recite to one another so that they might all hear the Bible in their own language. Spain and France were killing and exiling those who taught against Rome, and that’s how John Calvin (a Frenchmen) ended up in Geneva, where he wrote the first comprehensive systematic theology textbook for instructing new Christians.
All of this came to a head when Rome called a council to deal once-and-for-all with the reformers. This was the notorious Council of Trent.
Third, Rome formally and officially condemned all Protestants.
It is a historical and present fact that the Roman Catholic Church has formally set itself against Protestants, and it has never pulled back from that clear and official statement. At the Council of Trent, Rome condemned to hell anyone who believes some of the most fundamental doctrines among all Protestants. The canons or decrees of Trent anathematize3 anyone who believes that the Bible is the chief authority over all tradition and papal decrees. They also condemned anyone who believes that sinners are justified by grace alone through faith alone in the person and work of Christ.4
These statements are clear, they are recorded for anyone to see in the records of that council, and they are repeated in the Roman Catholic Catechism that is still used by Rome today.
It seems to me that Protestants and Roman Catholics can indeed be friends today. But it also seems to me that we must all recognize the differences between them are not mere preference nor are they minor. As I said above, though, it is likely that most Roman Catholics do not know the official teaching of Rome, just the same as most people who claim to be Baptist do not know what Baptists have historically taught.
Therefore, rather than using labels and throwing verbal grenades, I think we all might do well to simply have good conversations about the biblical gospel.
Endnotes
1. The Roman Catholic Church today shares many common doctrines with Protestants. These are not the doctrines that make Rome distinct as a Church. As time moved on, Rome increasingly articulated and demanded adherence to doctrines and organizational structures that are clearly absent from Scripture.
2. This article by Joe Carter does a good job of summarizing some of the main points of the Council of Trent as a historical moment that continues to impact Protestants and Roman Catholics today. https://www.thegospelcoalition.org/article/9-things-you-should-know-about-the-council-of-trent/
3. Anathematize is a fancy word that refers to a religious condemnation. The word anathema is a transliteration from the Greek ἀναθεμα. This was the word the Apostle Paul used to condemn any preacher of a false gospel (Galatians 1:8-9), and the word was and is used by the Roman Catholic Church to condemn anyone who opposes or diverts from official church teaching.
4. The key word in these doctrinal phrases is “alone.” Rome did teach and still teaches that faith in Christ is necessary for justification. The disagreement was never about the necessity of faith, but the sufficiency of it. Is a sinner justified before God by simply believing or having faith in the finished work of Christ? Rome says, “No!” Protestants say, “Yes!”
Holding the Line 
Thanks for the good article, Marc. I would add that, yes, Catholics regularly refer to “faith” and “grace,” but their understanding of those terms is markedly different from what the Bible teaches and what evangelical Christians believe.
Rome certainly teaches “that faith in Christ is necessary for justification,” but the “faith” that’s referred to is in the RCC and its sacramental system, NOT trusting in Christ as Savior. The Catholic catechism states, “By his death and Resurrection, Jesus Christ has “opened” (the gates” of) heaven to us” (CCC 1026), but it’s then up to each Catholic to walk through the gates via merit with the alleged help of the sacraments. That is not faith/trust as evangelicals understand it.
An excellent examination of the confusion caused by RC-ism’s misuse of Biblical parlance is “Same Words, Different Worlds” (2021) by Leonardo De Chirico. Link to Amazon below.
In the way you represent the Catholic Church as having no historical claim you somewhat gloss over over how much has changed in protestant churches compared to the time of the reformation. Those “protestant” churches would have looked esssentialy catholic to the modern protestant. Doctrines are not what separates modern Catholics and Protestants, most modern christans don’t even care doctrine outside of sola fide and sola scriptura. What truly separates us is the beliefs in apostolic succession and the trueness of the sacrements (mainly Holy Eucharist and Confession). Two things that existed immediately after the death and ressurection of Jesus. The apostles understood Jesus as instituting these things and went forth and practiced them. When the authority of Jesus’s church was originally regected, the protections for these sacred sacrements withered overtime within protestant churches. Regardless of why they originally split, I dont thing Martin Luther would be too please with the outcome of the Protestant church as it’s stray from Apostolic tradition. As for faith alone, Jesus did not say to the rich man “believe in me”, he said “sell your possesions and follow me”, in spirit and action.