What other beliefs, beside the Trinity morphed into their form of Christianity by the RCC have protestants also adopted?

Confession to a human priest, worship of saints and Mary: idolatry, institution or enforcement of Sunday worship, changed the 10 Commandments to fall in line with idolatry as practised in the RCC. The blasphemy that church rulings, doctrines or dogma are above Bible Scripture and or overrule them.

Roughly 95% of Catholic doctrine was continued in Protestant churches. They make all the same initial assumptions and tag different things onto the ends.

What did christian's believe before the catholic church existed?

The question is mute, since the Trinity didn't morph into Christianity. It was always part of.

You need a history lesson. The Catholic Church didn't exist till the Great Schism of 1054, when the Christian Church founded at Pentecost split in two -- the other half being the Eastern Orthodox Christian Church.

The Catholic Church is not a "form" of Christianity. It is genuine, complete, and original Christianity, the ONE Church Jesus Christ founded 2,000 years ago, the Church He said was to remain ONE, to which He promised "The Holy Spirit will guide you into all truth", and "Whatsoever you bind upon Earth is bound in Heaven", and "He who hears you hears Me". Every teaching of the Catholic Church has been part of Christianity for 2,000 years. All Protestant churches have adopted many teachings of the Catholic Church, or they wouldn't be Christian. However, each Protestant denomination has also rejected various beliefs of original and true Christianity, which makes them actually semi-Christian rather than fully Christian. Which is why the teaching of each denomination contradicts the teaching of the others. Truth cannot contradict truth.

the Bible (and the documents included within)
the use of altars
the use of church buildings (regardless of the name or title given for those buildings)
Sunday worship (not adopted by all Protestants)
sacraments (not adopted by all Protestants)
bishops (not adopted by all Protestants)
"saints" (not adopted by most Protestants)
"excorcism" of demons (not adopted by all Protestants)
etc. etc.

Protestants didn't get the Trinity from Catholicism, Protestants got the Trinity from the earliest New Testament Scriptures.

The concept of GOD the I AM, the 'GOD-MAN,' (Hebrew: Moshiach or Messiah), and the Holy Spirit as three distinct expressions of GOD first appear in Hebrew scripture. Deuteronomy 18 records the events surrounding an unusual request made by the nation of Israel in Sinai. They already had Moses as their prophet, but they asked for another Prophet to come to them Who was capable of embodying all of the powerful manifestations that they were seeing on the Holy Mountain, which were so terrifying that they constantly felt as though they would die. It was out of this mortal fear, they asked if GOD Himself could somehow come to them as a Man like Moses, since Moses they could tolerate without feeling mortal dread.

It was a desperate question, but Moses asked it on their behalf anyway. GOD did not laugh, become angry or explain that it was impossible. He simply said, 'The people have spoken well.' (Deuteronomy 18:17).

This began a series of prophecies, each describing this promised 'GOD-MAN' in greater detail the closer the time came for His arrival. This is clearly a description of Two Distinct Persons within the Godhead.

One of the prophets arriving later in the process spoke of the arrival of the Holy Spirit of GOD upon 'all mankind,' (Joel 2:28). This was a Third and unprecedented manifestation of the Godhead predicted to arrive after Messiah completed His work.

While your characterization is crude and obviously meant to contain sarcasm, it is actually accurate in the sense that the Christian faith, (in all of its forms), has been effectively 'grafted' into the tradition described above and every related doctrine, concept and belief contained within it can be traced back to these Old Testament prophecies and descriptions of GOD.

Catholicism is the One True Faith and a person can know this with certainty by reciting the rosary each day with care.