Yet this was how Christianity did develop in its early years, until 325 when the Nicene Creed (which I spelled incorrectly
) established the 'nature of God' as a Trinity. After then, yes, many will have been murdered as heretics. There's a good link
here which explains some of the early history, though there's vast amounts of early Christian history on the net which goes into this more deeply.
Since this afternoon, I've discovered that other Christian faiths which are unitarian (small u) include Jehovah's Witnesses, Seventh Day Adventists and certain Pentecostal churches. The essential difference between trinitarians and unitarians is that while the former worship Christ as a deity, as an aspect of God, the latter do not, but he is just as important. Both still worship God, he just feels a bit different.
Well, my position is still that we
don't know that Eru is Three or One. I would say that for Eru to be a trinity then we would have to have seen a Messiah on Arda in order for that to happen. There may
be a Messiah at some point in the future, but there hasn't been one as far as we know (unless maybe we start pondering Gandalf...
), so Eru is still One in terms of the timeframe of Arda we know about. In terms of our world I'm sure people did not start to become Christians before Christ appeared? That there has not yet been a Messiah could even hint that Arda is a Judaic world, though as I don't know enough about that, I'm not going to nail my colours to the mast on that opinion!
Maybe this is why Tolkien didn't want to see emblems of religion in fantasy! He knew we'd start arguing!