Building on my last post, it seems we can argue for the existence of a God that is not merely essentially trinitarian, but has all the essential features of the Christian God. Consider the argument below:

1. A necessarily existing God that has as its essential features all (and only) the essential features of the Christian God is metaphysically possible, i.e. exists in at least one possible world.
2. If (1), then there exists a God with all the essential features of the Christian God. (by S5 modal logic)
3. Therefore, there exists a God with all the essential features of the Christian God.

Now, this argument does not say anything about non-essential or accidental features of the Christian God, and thus perhaps does not quite deduce to the Christian God. But the argument, at least if it is sound (is valid and only has true premises), does provide evidential support for the claim that the Christian God exists.