What do you think of this argument for God?

[SuperSentient]

Yeah, I've heard this one before. I don't accept the first two premises to be true. The third premise is a guess that God exists in any possible world in the first place, so I can't accept that to be true either.

For the sake of the argument, it does depend on what God you're talking about. 

As for the argument:

1. God is the greatest possible being.

2. God is a necessary being, which means that God exists in every possible world (If God exists). 

3. If God exists in one possible world, God must logically exist in every possible world. 

4. Since God is the greatest possible being, it follows that every aspect of God (being possible) exists in some possible world.

5. Therefore, God exists (in all possible worlds, including ours).

I actually just structured the premises this way myself but is the same idea as an argument I heard before.

Number 5 is a wrong conclusion. You say in number 4 that every aspect of God being possible exists in some possible world. Suppose a God is characterized by some religion as someone that can levitate. According to what you say in number 4 only the possible attributes of the God can exist, since the ability to levitate is an impossible attribute in a possible world it follows that "God" exists without the attribute that God is originally defined with, which means that God doesn't exist.

Premise 4 says that every aspect of God is possible (exists in some possible world) since God by definition is the greatest possible being. By accepting premise 1 as a definition of God, all aspects of God are possible. Adding impossible aspects is inconsistent with premise 1.

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The first premise is true if the theist defines God that way (after all, you can't refute a definition unless it is logically inconsistent). The second just means that since God is necessary (must exist), the existence of God implies existing in every possible world (logically impossible to not exist).

I define an apple as a horse with bat wings and mad guitar skills. Do you accept my first premise? Shall I construct an argument about apples using this? While it may not necessarily be a flawed premise, it is a useless one if I want to make any real point with it. It should either be an obvious definition or one that I can demonstrate with previous premises. Where do we draw the line between definitions and bald assertions? This can apply to the second premise as well. I accept neither of them.

The rest of the argument also slips up at the word "possible". Theists aren't able to demonstrate the possibility of any of these things attached to that word here. Until then, this is a useless argument.

An apple has a universal definition.
You would agree though that god has no universal definition, it may be up to religions. Theists can use premise 1 as the definition of their god.