Best Answer:
During the rule of Constantine I, he brought the leaders of the Patriarchates together, for the purpose of voting on doctrines on which there were divisive disagreements that were tearing the Christian faith apart, and the Roman empire with it.
As a result, any leaders who disagreed with the majority decisions were declared to be heretics, excommunicated, and replaced by the decree of Rome. The chief disagreement among the leaders was Arianism vs. Trinitarianism.
To resolve the issue, the First Council of Nicaea, to which Constantine had called the religious leaders in 325 CE, issued a proto-trinitarian creed declaring that both the Father and Son were members of what is styled as "the godhead". In 381 CE, a new creed was authored during the First Council of Constantinople, declaring that The Holy Ghost, (aka Spirit) was also a member of the godhead. This creed has to this day, and for most of the history of Christendom, been passed off as the "Nicene Creed", though it was not authored in Nicaea.
After excommunicating any dissenters, the combined authority of the Roman Bishop and the Roman Emperor eventually became vested in one man, who was then declared to be inspired by "Apostolic Succession", and called, the Pope.
Briefly, and oversimplifying, the following happened:
Islam split from Christianity over doctrines established about trinitarianism and the death of Christ, about 700 CE.
The Orthodox Church split from Rome over who the Pope should be, about 1054 CE.
Protestants split from Rome over whether there should be a Pope, during the 16th to 17th centuries.
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