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I see what you're saying that logically, there can be no power outside of the all-powerful to challenge the all-powerful, otherwise the all-powerful would not be all-powerful. You're talking of God and suggesting limits to His power and what that means for His existence.

The proposition you venture to blow up with the aid of the above logic, correct me if I'm wrong, is that omnipotence necessitates immortality. And, correct me if I'm wrong, you conclude that if omnipotence must but cannot necessitate immortality, no being can be all-powerful.

I think you confuse one of the two facets of the argument, which is that omnipotence necessitates immortality.

The all-powerful cannot not be immortal. For omnipotence exists and can only exist beyond its manifestation in the life cycle of mortal existence, b/c it would not be all-powerful if it was subject to Nature's power.

But since all things have power, isn't it logically impossible for a being to possess in itself all power? If we're speaking solely regarding power in mortal existence, yes. However, as we said before, nothing mortal can be omnipotent anyways.

Far from redundant, this implies the all-powerful also must be all things, if no power may exist outside itself. Yet, if the all-powerful is not of mortal existence, it still must possess Nature's power as its own. Then it can only compass Nature as omniscience.

But how can an omniscient, omnipotent being exist who is not of mortal existence, of Nature's underlying fabric of the kosmos? As the overlying fabric of the kosmos; omnipresent.

Thus this being evinces the triumvirate aspects of singularity: omnipotence, omniscience and omnipresence. And b/c it is a singularity it cannot become less powerful and cease to exist by turning against itself. All its power is contained within itself and singularity is not divisible.

Once it is it cannot logically be otherwise. So the real existential question to ask is why God ever came to be, not whether He could exist. What a miracle it is!