I suppose that when people say that if God exists she exists necessarily, they mean that if she existed there would be no possibility of her not existing. Unlike everything else in the universe, e.g. me, sub-atomic particles, the galaxy, anti-matter etc.

I'm not sure how one would argue for or against the notion that the lowest prime number (2?) exists necessarily, in all possible worlds. I know some people believe that mathematical statements such as '1=1=2' are necessarily true (in all possible worlds?) - or does that raise different issues?

But I agree that in any case one should not try to use the lowest prime number as a foundation on which to erect an ethical system.
('He treated her in a shamefully un-two-ist way'?)

No doubt these are the sort of questions that people will be chatting about on the inter-faith walk.