Hi Allan,
It’s not ‘doubt’ that is fatal to either argument per se. It’s about arguing one’s case consistently and with intellectual integrity. The debate over God is a debate over whether or not God is a metaphysical possibility—i.e., whether ‘God exists necessarily’ or ‘God does not exist necessarily’ is true.
Give our limited knowledge and faulty reasoning, any of us could be wrong, right? We all admit we are less than omniscient and so might be wrong in our belief that ‘God exists necessarily’ is true (or that ‘God does not exist necessarily’ is true). What Hartshorne is forcing atheists to do is argue the ONLY possible case for atheism, viz., that ‘God does not exist, necessarily’, that is, that God is impossible, that no such being can possibly exist; and to the extent one doubts this, i.e., to the extent that one grants that God is a real possibility, one must grant that God in fact exists because those are the only options.
It’s either:
(A): “a necessarily existing God is a metaphysical possibility”
Or
(B) “a necessarily existing God is a metaphysical impossibility.”
Atheists have to argue (B) to be atheists. It’s the only credible and honest atheisitc position there is. They cannot say “Well, sure, there’s nothing logically or metaphysically impossible about God’s existence. But so what? God does not in fact exist” (because there is evil or whatever the atheist wants to offer as reasons for his unbelief), for to argue this way is self-contradictory. To argue the *metaphysical possibility *of some *necessary being *is to argue for that being’s actual existence. The only way a particular ‘necessary being’ can be said to not exist is if it is said to not exist necessarily. And to not exist necessarily is to be ‘impossible’. A ‘necessary being’ can’t ‘not exist’ contingently. See?
Just make sure you keep ‘epistemic’ possibility separate from ‘metaphysical’ possibility.
Tom