Why Does God's Possible Existence Mean He Must Exist?

INTERVIEWER: I want to move on to the ontological argument. I kind of understand that if a maximally great being exists, then he exists in every possible world because that's what a maximally great being would need to do if he was maximal. But why, if it's possible that he exists, that he must? That's the part I don't understand.

DR. CRAIG: OK. You know, it's so funny you should ask that because just before we completed our Skype call I was reading Anselm's Proslogion about the ontological argument. The key idea here is that in order to be maximally great, this being must be not only omniscient, morally perfect, omnipotent, but he must also have necessary existence because it's greater to exist necessarily than merely contingently. So what that means is that if God exists in any possible world, then he has to exist in every possible world because that's what necessary existence means. Necessary existence is to exist in every possible world. So either God either exists in no possible worlds, to say he's impossible, or he exists in every possible world. So if God exists in one possible world, he exists in all of them. Now another way of saying that is to say if God's existence is possible, then it follows that God exists. So what the atheist has to say is not simply that God does not exist; the atheist has to say it's impossible that God exists. Because if he exists in any possible world, if it's possible that he exists, then he exists in every world, including this one. So the atheist is a lot more radical than we think. It's not just that God doesn't exist; it's impossible that God exists. So I just invite people. I don't try to prove to them that God's existence is possible. I just say what do you think? Do you think it's possible that God exists, or do you think it's impossible that God exists? And I think the concept of God is a perfectly coherent, possible concept, and therefore I think this is a good argument for God's existence, remarkably.

INTERVIEWER: It's that bit where we jump from that if it's possible that God exists, he exists in every possible world. I can understand that, but...

DR. CRAIG: Well, now wait. We don’t need to make that jump. You made a jump there. If it's possible that God exists, then he exists in a possible world. If it's possible that he exists, then he exists in at least one possible world. That's what it means to say that something is possible. If I say it's possible that you have red hair, that means there is a possible world in which you have red hair. Or if I say it's possible that I weigh 300 pounds, there's a possible world in which I weigh 300 pounds. So if you say it's possible that God exists you're saying there's a possible world in which a maximally great being exists. And then the next move is to say, wait a minute, if he exists in one world, he exists in all of them because a maximally great being has necessary existence, not just contingent existence. He can't just exist in some possible worlds but not all of them, otherwise it's not necessary existence. So if there's a maximally great being in one world, he's in all of them because he has necessary existence.