Why can't you exist in one universe and not in another? Isn't this the obvious implication of multiple universes? How can a virtual particle appear and disappear? Wouldn't the fact that you can appear mean that your disappearance is not fatal to your reappearance?
Belief in Him causes Him to communicate with you - like picking up the phone and making the first call.
There is no difference between saying non-believers don't exist to God until they believe in Him, and saying God doesn't exist to non-believers until they believe in Him.
How so? If God doesn't exist to the non-believer than the non-believer doesn't exist to God.