I might quibble about the Catholic Church being the "original" church since Catholicism only came about after Theodosius I made Christianity the official religion of the Roman state in 380. You could argue that Catholicism started a bit earlier under Constantine I at the First Ccouncil of Nicaea in 325, which is when the Roman state started to consolidate the various early Christian beliefs under an official "catholic" orthodoxy. The word "catholic" literally means "including a wide variety of things". The point being that there was already a wide variety of Christian sects prior to the Council of Nicaea.

The Protestant argument against Catholicism boils down to the belief that the Catholic Church is a corrupted Christianity, not that it is non-Christian. And there is some truth to that. The pre-Nicaean churches were free-wheeling spiritualists with a wide variety of beliefs, but that all changed when the Roman state decided to create an orthodox, singular religion under its control. Protestants argue that this adaptation of religious belief to the needs of maintaining state power is the original corruption of the Catholic Church.

Now, two key facts influenced the early history of Roman Catholicism:

1. The Roman state recognized the descendants of Caesar, the Emperors, as the Pontifex Maximus, or head priest, of the Roman state. They also required that everyone adhere to the cult of the Emperor. This was purely ritualistic and was meant as a bulwark to the power of the state.

2. The vast majority of the Empire's citizens were pagan.

Because of #1, the Roman Emperor became the head of the newly formed Catholic Church, which was a unification of Church and State. This is called Caesaropapism, and is also why the Catholic Church retains a hierarchical structure to this day and its seat is still located in the heart of the Western Roman Empire. The Pope is the spiritual successor of the Western Roman Emperor.

Because of #2, Catholicism is highly ritualistic, like paganism, and early Catholicism adopted the worship of saints, which are basically small gods. Saint worship was the bridge between paganism and Christianity.

During the Protestant Reformation of the 16th century, Luther and others made the point that the union of state power with Christianity was a corruption of "original" pre-Catholic Christianity, which was more spiritually-oriented and valued personal conviction over state orthodoxy. Interestingly, the split between Protestantism and Catholicism in Europe also more or less follows the geographic outline of the Western Roman Empire, with southern Europe largely retaining Catholicism and northern Europe largely adopting Protestantism. This implies a political dimension to the schism, not just a religious one. England is the odd man out here because their response to the schism was to create the Church of England, which is basically Catholicism without the Pope, substituting the English monarch as the head of the Church and toning down the saint worship.

The great irony of any Protestant movement that craves Christo-fascist state power is that they are advocating to become the very evil they swore to destroy back in the 1500s.