This same basic OP question was beat to death in the prior thread named Which denomination today is closest to 1st Century Christianity. (That thread was closed because the posting became uncivil.)
Though I am not tempted to believe that the oldest schismatics today are the original Christianity, still, I do believe the various old christianities tend to have many wonderful historic echos and doctrinal debri from early christian themes. I also think a study of the oldest schisms from rome, Africa, Byzantium, etc, are wonderful sources for historians trying to understand HOW certain doctrines were created, changed and how some were lost.
1) A Loss of prophetic leadership
I do not think that doctrinal accretions and evolutionary changes to the earliest doctrines was completely unavoidable. For example, once the last authentic apostle, John the revelator, died, then, all congregations such as Antioch, Rome, Jerusalem, Galatia, ephesus (and others not mentioned in NT texts...) were in the position of having no living source of apostolic authority nor prophetic revelatory guidance such authentic leaders [αποστολοι, etc.] had provided in the fledgling Jesus movement.
2) Early Doctrinal Immaturity of the Jesus Movement
Once the early congregations were "on their own" to make their own way as best they could without revelatory leadership it was inevitable that all would tend to stray in differing doctrinal directions.
Origen pointed out that the Christianity of his age had not yet decided whether God the Father had a physical body or not. Such a doctrinal immaturity was a difficult situation since many other types of doctrinal details were not spelled out for the christian movement which was coming under increasing pressure for answers to such questions from honest investigators and their antagonists alike.
3) Christianity attempting to better define itself
This lack of answers to simple doctrinal questions led to many early arguments and to a proliferation of multiple theories and further schisms were not unexpected. The ancient arguments over doctrines often reminds me of the very, very similar arguments we all see in modern religious and philosophical forums.
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