Some thoughts/questions to the single steps of your argumentation.

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»» A) There is nothing greater than All that exists. Together we can reduce this set to the unit one. All.
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What do you mean with these words? What does "greater" mean, what does "All that exists" mean. How do you disprove the "existence" of the 2 different natures (following certain christian philosophers) with no common areas, transcendence and immanence? Why should I agree to reduce this to an unit? Does this unit means a real-definition of a really existing connection between "all that exists"? Or is this merely a wish of you? To me your step A isn´t but a combination of words which might or might not refer to reality.

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»» B) We are created from within that set of All.
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What do you mean with "creation"? Isn`t this a pre-decision concerning the question wether we are created or not? Would you call a self-regulating system a "creation" (which term implies the existence of a creator)? Did you read about the "imaginary time"-theory of Hawkins which evaporates the necessity of a creator.

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C) We are subject to the laws and actions from within that set of All.
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And? What does this imply? Do you think your step D is the logical and incontingent conclusion of C?

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D) All is cognitive as we are cognitive and are contained within the set of All.
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May be, you wish this to be real. How do you proove your cognition being the same as a rat´s one or the moon´s one? How do you proove the moon being cognitive at all? Or isn´t the moon part of "all that exists"?

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E) The majority of monotheists define God as the "Supreme Being".
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And so you think that questions of "real existence" are questions of democratic voting? Because the "majority of monotheists" thinks that God exists, God exists? Because they define God as the "Supreme Being", such a Being exists?

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Conclusion: God is All and any lesser force no matter how great within All is not Supreme.
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Conclusion: Words, word, words witout any certain meaning

Roger