The Emblems of Religion don't belong ... or do they?

[Aiwendil]

Lalwende wrote:

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... )

Of course, while Gandalf may be a kind of Messianic figure, he is emphatically not a "person" of God.

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?

Yes, but once that's conflating two issues. Christian theology (excluding the unitarian sects you mentioned) holds that God was always a trinity, even before there were any Christians.

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!

I think that the "Athrabeth" is rather strong evidence that Tolkien considered, at least at one time, the incarnation of God to be a definite part of Arda's future. Of course, there is also a Messianic tradition in Judaism.