Possibilities have no actual existence in their own right. They concretely exist only in and as potential acts of some concrete actuality or other. And possibilities exist necessarily: for x ever to be possible in any state of affairs, x must in every state of affairs always have been possible in and for some state of affairs or other. Possibilities exist eternally. That possibilities necessarily and eternally exist means therefore that in every possible state of affairs whatsoever there exists an eternal, necessary concrete actuality, in whom all possibilities exist as potential acts.
If anything be possible, then, God necessarily exists.