The Ontological Argument - valid or debunked?

[GrandizerII]

We know that there's something wrong with that logic because it can "prove" that gods do not exist just as easily as it "proves" that they do:

Premise 1: It's possible that a 'maximally great being' does not exist. 
Premise 2: If it's possible that a maximally great being does not exist, then a maximally great being does not exist in some possible world.
Premise 3: If a maximally great being does not exist in some possible world, then it does not exist in every possible world. 
Premise 4: If a maximally great being does not exist in every possible world, then it does not exist in the actual world. 
Premise 5: Therefore a maximally great being does not exist in the actual world.
Premise 6: Therefore a maximally great being does not exist.
Conclusion: Therefore God does not exist.

This second version is exactly as strong as the first version, yet it proves the exact opposite.  Any argument that proves both X and not-X is worthless.  In the scales of persuasion, its weight is zero.

We know, therefore, that the MOA (modal ontological argument) is worthless, absolutely refuted. 

Which brings us to P1.   

We're talking about necessary gods, gods that don't exist at all unless they exist in all possible worlds.  Can a god like that exist in some possible worlds? 

No.  Certainly not. 

We know that some possible worlds are godless.  We know this because of the definition of "possible world."  A possible world is any world without contradictions.  If a world doesn't have square circles or married bachelors, or anything other logical contradiction, then it is a possible world. 

There's nothing contradictory about godless worlds, so they are by definition possible. 

But a god existing in a godless world would be a contradiction, an impossibility.  Therefore, no god can exist in all possible worlds.  

Since P3 establishes that the god we are discussing, the god of the modal ontological argument, does not exist in any possible world unless it exists in all of them, it follows that this god does not exist in any possible world. 

Therefore, P1 is false. 

I'm sorry, but it makes no sense that this defeats the argument. What is the difference between Premise 1 and Premise 1'?

Premise 1: It's possible that a 'maximally great being' exists.
Premise 1': It's possible that a 'maximally great being' does not exist. 

So the real difference is in P4' (P2' and P3' are definitionally true). 

Premise 4: If a maximally great being exists every possible world, then it exists in the actual world. 
Premise 4: If a maximally great being does not exist in every possible world, then it does not exist in the actual world. 

When you apply the modal logic "if something is necessary in one possible world then it is necessary in all possible worlds" to a negative such as P4', you are saying it is necessary that something does not exist. Isn't that saying the same thing as a maximally great being is logically impossible? So to support P4', you are back to having to show that a greatest conceivable being is not logically possible--which was the original challenge of the original argument. 

wiploc's argument shows that it can work both ways, that's the point. As you have yet to provide an argument that establishes the logical possibility of a maximally great being, then this argument is pretty much vacant at this point.