A modal argument for God's existence - why it fails

Philosophers of religion out there - is there a version of a modal ontological argument which isn't vulnerable to the charge that the premise:

 1. It is possible God exists

just trades on the ambiguity between metaphysical and epistemic necessity/possibility? Maybe there is, but I have not spotted one yet given an admittedly brief survey.

Here is a simple modal argument for the existence of God:

1. It is possible God exists
2. If it is possible God exists, God exists at least one possible world
3. If God exists at one possible world he exists at all of them (being a necessary being)
4. Therefore God exists at every possible world
5. Therefore God exists at the actual world

Good argument?

Note first of all that if we have good grounds for supposing God does not exist at the actual world, then the logic of the above argument also requires that we have good grounds for supposing God does not exist at any possible world. That is to say, we have grounds for thinking it is impossible that God exists. And perhaps we do have such grounds - e.g. in the form of vast swathes of evil.

In response, theists may say: 'Ah but it is possible God exists, because I can imagine God existing - I can certainly imagine that the property of maximal greatness is instantiated, say.'

Three key points to make when debating the existence of God

Three key points to make when debating the existence of God.

1. Defining God

First, in asking: Does God exist? It would be good to get some clarity about which God we are talking about.

I shall assume we are talking about a God that is omniscient, omnipotent, and perfectly good:

Prof William Lane Craig defines God as a 'maximally great being' - which he says requires that God be morally perfect.

Prof Richard Swinburne similarly characterises God as 'a person who is omnipotent, omniscient, and perfectly good'.

It suffices to establish atheism, then (given these guys' characterisations/definitions of theism), that I show beyond reasonable doubt that there's no being that is omniscient, omnipotent and perfectly good.