I use the term conditional theistic monism (CTM) for the view that if god exists, only god exists. I’m not sure I can think of a header image for this one. Here’s the argument.

1. What matters in the survival of a person is Relation R (following Parfit) — strong overlapping chains of strong psychological connectedness between mental states by any reliable cause.
2. What matters in the survival of a person is all that matters in the existence of a person.
3. All that matters in the existence of a person is Relation R. (from 1 & 2)
4. Knowledge of what it is like to have any mental state requires possession of that mental state.
5. God — an omniscient being — would know what it is like to have all mental states.
6. God would possess all mental states. (from 4 & 5)
7. Possession of all mental states entails possession of all psychological connections between those states.
8. God would be a reliable cause for psychological connectedness between mental states.
9. God would sustain all instances of Relation R. (from 6, 7 & 8)
10. God’s existence would be a sufficient condition for everything that matters about the existence of all persons. (from 3 & 9)
11. All that could matter about the existence of the universe is that it causes persons to exist.
12. If God exists, the existence of the universe does not matter. (from 10 & 11)
13. God would never act arbitrarily — that is, without good reasons.
14. If it doesn’t matter whether the universe exists independently of God, there would be no good reason for God to create the universe.
15. God would not create the universe. (from 12, 13 and 14)
16. Therefore if God exists, only God exists. (entailed by 15)

This argument won’t convince many classical theists, but I’m still curious to what extent a classical theistic conception of god as omnipotent, omniscient, omnibenevolent, and eternal or everlasting could be consistent with CTM.