Plantinga has followed in the steps of Leibniz, who stated that if God possibly exists, then God actually exists. This is because God is defined as a being who possesses all great-making properties. So, God either necessarily exists or is impossible. The Modal Ontological Argument goes like this:
1. It is possible that a maximally great being exists.
2. A being is maximally great only if it is maximally excellent in all possible worlds.
3. A being is maximally excellent only if it is omnipotent, omniscient, and perfectly good.
4. Hence, a maximally excellent being exists in all possible worlds.
5. Therefore, a maximally excellent being exists.
The four great-making properties listed are: omnipotence, omniscience, perfect goodness, and necessary existence. The first three are covered by what Plantinga calls "maximal excellence." Only if a maximally excellent being exists necessarily is it maximally excellent.
Now, Kant objected that existence is not a predicate. However, the proponent of the MOA can grant this and go merrily on his way. For, existence is not assumed to be a great-making property anyway. Rather, it is necessary existence, as opposed to contingent existence, that is deemed a great-making property. Kant's objection, therefore, while interesting, is irrelevant.
Yet, if a maximally great being (God) is possible, then it must exist. For, if God were to exist only in some possible worlds, then He would be contingent and not necessary, which means He is not maximally great. That, of course, is contradictory.
The opponent of the MOA can only conclude that God (defined not just as "First Cause" or "Creator," but as a maximally great being) is impossible. In fact, if one starts a counter-argument like this:
1*. It is possible that a maximally great being does not exist
it follows that a maximally great being does not exist.
The only two options, then, are that a maximally great being necessarily exists, or else impossibly exists. A maximally great being cannot exist contingently, for then it would not be maximally great.
Is there any compelling reason to believe (1) or (1*)? Robert Maydole's recent formulation of the ontological argument - what he calls the "Modal Perfection Argument" - attempts to establish the possibility premise. With some word-substitution, I believe we can transform Plantinga's rationally acceptable argument into a compelling proof.
A1. If a property is a great-making property, it's negation is not a great-making property.
A2. If a property A is a great-making property and the property B is a necessary condition for A, then B is a great-making property.
A3. Being a maximally great being is a great-making property.
Now, the proof that a maximally great being is possible:
P1. If it's not possible that a maximally great being exists, then every being has the property of not being maximally great.
P2. If every being has the property of not being maximally great, then not being maximally great is a necessary condition.
P3. If not being maximally great is a necessary condition, then not being maximally great is a great-making property. (From A2)
P4. Not being maximally great is not a great-making property. (From A1 and A3)
P5. Therefore, it is possible that a maximally great being exists.