Ok I know that this is a very complex topic and Christianity has a lot of denominations but as an attempt to simplify it, is it safe to say that Christians can be divided into two categories (before dividing them more), those that believe in the Protestant reformation and those that don’t?
Is It safe to say that those that don’t are some form of catholic?
If you were to create a tree diagram with Christianity in top, what would the next branch be? For me, it would be Protestant reformation vs catholic.
The Orthodox and Catholic churches divided long before the Reformation. Additionally, you had those churches controlled by Rome in the 4th Century and those outside of Roman Control. The Roman government enforced the decisions of the various councils, and so forced certain theological viewpoints and liturgical practices upon Christendom within the Roman state. Those outside of Roman control continued to hold views and practices contrary to the councils until they were subsequently conquered and were made to follow the beliefs and practices.
For instance, Arianism and various other Christologies were deemed heretical by the councils, but such beliefs persisted outside of Rome at the time.
If you go down and look at the Church Fathers, there was plenty of division in thought amongst them as well. Justin Martyr, a very influential theologian in the Early Church, maintained that Jesus was a second, lesser god.
And if you start to include all of those rejected by the Catholic/Orthodox Church, like the Gnostics and Marcionites, there are very many branches indeed.
Part of answering such a question then becomes: by what criteria do you include or exclude groups that identify themselves as Christian? Belief in the Jewish God? Belief in the Jewish God as good? Belief in the Jewish God as the only God? Belief in Christ as a Prophet? Belief in Christ as Messiah? Belief in Christ as God himself? Belief in Christ as a personage of God? etc.