Not trying to disrupt, just honestly want to know...What IS 'the protestant church'?
The Protestant church was started when some of the Catholic church broke away do to disputes in the teaching that was taking place within the church. That is how the name got started, because they broke away and protested the apostate teaching !!!
From there for some reason the Catholic church tries to state that all denominations are Protestant just because they each point out some of the bad teachings of that church that do not align with the word of God.
Augustine was one of the first ones who started the apostasy as he was influenced by outside sources and started the eternal security doctrine in the 4th century. Where he stated that apostasy and falling away was impossible, which is contradictory to what the bible says.
From here the Romans corrupted the teachings of the Church (body of believers) and from within that the RCC was formed.
From the first 3 centuries there was no such thing as the Catholic church, and even in the 2nd century (101-200 AD) when the term catholic was first used to describe the Church. It was only used to explain its universal aspect that anybody no matter what race or nationality could be a Christian.