The standard modal ontological argument (e.g., in Plantinga, with roots in Leibniz) says basically:
- Possibly, necessarily God exists.
- Therefore, necessarily God exists.
- Possibly, the proposition that God exists is necessary relative to q. (Premise)
- Therefore, the proposition that God exists is necessary relative to q.
- q holds. (Premise)
- Therefore, God exists.
So, to make a modal ontological argument work, we don't need to establish that possibly necessarily God exists. All we need to establish is that possibly, God exists relatively necessarily, relative to some true proposition q.
One plausible candidate for q is the proposition is that I exist. For it might be that I find it conceivable that I essentially depend on God for my existence, and conceivability is evidence for possibility, so possibly I essentially depend on God for my existence. Thus, possibly, God necessarily exists (relative to the proposition that I exist), and hence God exists.