"Catholics aren't real Christians"
The above was said by a Christian, not that all Christians believe that statement is truthful, or that only Christians believe that statement is truthful. Well, aside from most Christians possibly not being real Christians, here's the problem with that statement ... all modern Christianity stems from the Catholic Church.
Firstly, the book Christians use to claim their version of Christianity is the right version of Christianity, was put together by the Catholic Church. After Constantine the Great accepted Christianity as his religion, and declared freedom of religion in the Empire, he ordered church leaders to agree upon a single canon that would be used by all Christians, and financed 50 copies to be made. This led to the creation of an official Bible, with them deciding what would, and wouldn't, be in it.
Secondly, after the Council of Nicea, in 325 AD, when it was decided that Jesus was God, and the opposing Arian view, that Jesus wasn't God, was deemed heresy, the Nicene Christians began persecuting other Christians. The irony, as I see it, is that Arianism was most prevalent in the East...where Christianity originated. Constantine may have also seen the irony, as he chose an Arian Christian to baptise him, before his death. In 380 AD, Nicene Christianity was made the state religion. This led to further persecutions, and attempts to eradicate all forms of Christianity that the Church of Rome deemed heresy. Arians were their main target, initially.
By 415 AD, Arianism had mostly been suppressed within the Empire, so the Church of Rome turned its eyes on a new growing Christian denomination, Pelagianism. Pelagian believed in deeds over grace, even going so far as suggesting that good people, who do good works, can go to heaven, even if they don't go to church, or if they practice another religion. He also believed that by Jesus dying for everyone's sins, people were no longer burdened by original sin...that they are born good. Pelagianism started attracting many followers, so the Church of Rome deemed it heretical at the Council of Carthage, in 418 AD.
This persecution and suppression of all other forms, or denominations, of early Christianity led to the Catholic Church being the only widely practised form of Christianity for hundreds of years. Crusades were aimed not just at Muslim nations, but also nations deemed heretical. The inquisitions were aimed at anyone not practising Catholicism. All modern forms of Christianity stem from Roman Catholic roots, and the Bible they put together.