You can watch William Lane Craig giving the Ontological Argument for the existence of God on YouTube here:

https://www.youtube.com/watch?v=dr36HID62wM

This argument is what I call the 'Neverland supper' of theistic arguments - as with the lost boys' food in Peter Pan, Craig is trying to manifest God by closing his eyes and using his imagination to believe it into existence.

This is the syllogism in Craig's words verbatim from this video:

Premise 1: It's possible that a 'maximally great being' exists.
Premise 2: If it's possible that a maximally great being exists, then a maximally great being exists in some possible world.
Premise 3: If a maximally great being exists in some possible world, then it exists in every possible world.
Premise 4: If a maximally great being exists in every possible world, then it exists in the actual world.
Premise 5: Therefore a maximally great being exists in the actual world.
Premise 6: Therefore a maximally great being exists
Conclusion: Therefore God exists.

So, let's have a look at this.

Premise 1: It's possible that a 'maximally great' being exists.

Craig is conflating a possibility with a hypothetical idea. Hypothetically you can posit the idea that a 'maximally great being exists' but that in and of itself doesn't make it possible. If you want to say that something is possible, the burden of proof is on you to demonstrate that it is possible. All Craig does is assert that it is, with no evidence and flawed logic. As Hitchens rightly says, anything that can be asserted without evidence can be dismissed without evidence.

Premise 2: If it's possible that a maximally great being exists, then a maximally great being exists in some possible world.

We can't get to 2 because 1 is flawed, but for the sake of argument - again, Craig is making the same mistake (I am giving him the benefit of the doubt that it is an intellectual error rather than a conscious deception) confusing the idea of a hypothetical reality, which only exists in the mind of the person thinking about it, and a possibility as being something that might exist in some alternate reality. Again, bald assertion, no evidence, faulty logic.

Premise 3: If a maximally great being exists in some possible world, then it exists in every possible world.

This is contingent upon Craigs own definition - which literally means that is so because he says so. What is a possible world? Does he mean an alternate universe in the multiverse? Or does he mean an abstract hypothetical - because the latter does not exist, only the idea of it exists in Craig's mind.

Premise 4: If a maximally great being exists in every possible world, then it exists in the actual world.

Another bald assertion and non-sequitur. All we have here is Craig's say so. It's the same fallacy of conflating 'possible worlds' with alternate realities or hypothetical ideas. He's trying to manifest God directly out of his imagination.

Premise 5: Therefore a maximally great being exists in the actual world.

This is where the supper manifests directly from your imagination into your bowl... which is great unless you actually have an appetite for real truth.

Premise 6: Therefore a maximally great being exists

That's not really a 'therefore' as it's implicit in the previous premise, he could have skipped this one.

Conclusion: Therefore God exists.

And here's a rabbit I've pulled straight out of my ass! Let's call him Zeus!