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Which sentence would be correct?

" it is not my merit, but that of He who has created me and who still watches over me"
" it is not my merit, but that of Him who has created me and who still watches over me"
Thank you!

ps. If there are other mistakes please feel free to correct me
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Use "of him," object of the preposition.

Notice that in the following clause, who has created me, "who" is the subject and "me" is the object.

I know it's confusing, because "him" and "who" represent the same person. "Him" is objective and "who" is subjective. (But "who" is the subject of the clause.)
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I agree.

You must always remember that after a preposition, you can never have a subject pronoun, just an object pronoun (me, you, him, her...)
"You" is both subjective and objective.

You gave it to me, and I gave it back to you. [H]

(He gave it to me, and I gave it back to him. I gave him a dollar and he gave me a nickel.)