Argument

1. A perfect being exists.
2. A perfect being would be able to be the Christian God.
3. If (2), then there is a possible world in which a perfect being is the Christian God.
4. If a being is the Christian God in one possible world, then it is the Christian God in all possible worlds.
5. Therefore, the Christian God exists and is a perfect being.

Now, admittedly, this may not be the most persuasive way to reason from the existence of a perfect being to the existence of the Christian God. But is seems to be one way to do so.