A Necessary Being?

[Neo-Scholastic]

Here, the same logic proves that god does not exist. 

Premise 1: It is possible that God does not exist.
Premise 2: If it is possible that God does not exist, then God does not exist in some possible worlds.
Premise 3: If God does not exist in some possible worlds, then God does not exist in all possible worlds.
Premise 4: If God does not exist in all possible worlds, then God does not exist in the actual world.
Premise 5: If God does not exist in the actual world, then God does not exist.
Conclusion: Therefore, God does not exist.

Premise 3 does not follow. If you have ten rooms and only one hasn an apple in it then the apple exists. But if instead all but one do not have and apple then an apple still exists...in the one that does not not have an apple.