This is an argument I just can’t make work, but I can’t shake the idea there is something to it.
If the principle of causality is that all potencies are brought to act by something else, AND act and potency are understood as reducing existence and essence, then the possibility of causality will rest on the possibility of an essence identical with existence.
Take the 24 Thomistic theses. It seems like they want to base the whole system on the division of pure existence from composites of essence and existence. If even causality is to be understood in light of this distinction, it seems to me what we are saying is that the recognition of (at least the possibility of) an essence that is the same as its existence is analytically prior to our notion of causality. The recognition of the possibility of pure existence is thus presupposed to the account we give of causality.
The upshot of this is that causal axioms ultimately reduce to the recognition of the possibility of necessary existence i.e. essence identified with its existence. A theistic proof thus amounts to this: causes are actual, so the basis of causality is actual; but the basis of causality is necessary existence.
I find this interesting because if theistic proof really rests on the possibility of existence/essence identity, then it seems we should be able to make a contemporary ontological argument work if a cosmological one does; or perhaps, a la Kant, we could deny essence/existence identity and take it as denying the basis of cosmological arguments.