From what I understand:
Christians were persecuted (killed) in Rome for those 300 years. You can look up the catacombs and those old underground churches (very cool). One of the pillars of my faith is the martyrs who died for their beliefs.
Some churches rejected the Nicene Creed - but it wasn’t a majority (or a lot). I enjoy reciting the Nicene Creed and believe it to be true, but I know some may disagree.
I'm pretty sure it was a lot. Probably not a majority, but enough to be useful to the Roman state. States are built on violence, which means for a state to justify its existence, it must have enemies, and it must manufacture enemies if that's the most convenient way to grow itself.
But the reason I added that was to lead you or whoever reads it to the conclusion that early Christianity was distinctly different from what the Orthodox made it, thus refuting (perhaps preemptively) their assertion that they maintain the pure version of Christianity, which Jesus would recognize - I've heard them say that repeatedly, so I think that's part of their sales pitch. Its simply not true, and is actually ludicrously impossible.
