There is often confusion about God and possible worlds. If a being is contingent, it means it exists in at least one possible world. It may perhaps exist in more than world—indeed, in many worlds—but the key is that it will not exist in every possible world. There will be at least one—probably many more—world in which it does not exist. If a being is necessary, then it exists in every possible world. Also, if a being is construed as necessary, then its existence is either impossible to be false or just plain impossible.
This has definite application to God. Of course, if God is contingent, merely showing that he exists in some world or other, or showing that he does not exist in one world, doesn’t really accomplish much (except, of course, if the world under consideration is the actual world!). However, if God is construed as necessary, then showing that he doesn’t exist in some possible world is tantamount to saying that he doesn’t exist at all. This is because something that holds its existence as necessary either exists in every possible world or in no possible world. Lacking existence in one possible world entails not existing in every possible world; therefore, a necessarily existent God who does not exist in one of the possible worlds does not exist in any of them—his existence is impossible. Of course, God’s existence could always be construed as contingent, but not without strong theological cost.
However, it’s also important to note that this means that if God’s existence is even possible (that is, if there is even at least one possible world in which God exists), then he must exist, and his non-existence is impossible. So, if it can be shown that God’s existence is possible, then every possible world is populated by God. So what does this all mean? This means that God’s existence is either necessary or impossible. So the next time an atheist tries to use possible world semantics to show God doesn’t exist (this doesn’t happen too often, but sometimes it does), unless he shows God’s existence is impossible, it simply won’t affect your conclusion!