W-rV.in.9. Let this review be then your gift to me. 2 For this alone I need; that you will hear the words I speak, and give them to the world. 3 You are my voice, my eyes, my feet, my hands through which I save the world. 4 The Self from which I call to you is but your own. 5 To Him we go together. 6 Take your brother's hand, for this is not a way we walk alone. 7 In him I walk with you, and you with me. 8 Our Father wills His Son be one with Him. 9 What lives but must not then be one with you?

9 What lives but must not then be one with you?

Understanding this as meaning “There cannot be lives that do not have to be one with you”….

Should that understanding be possible?

Either the verb or the plural noun, but I am not quite happy with your paraphrase. I would choose:

All lives /All that lives must be one with you.
In old writing, "but" can also have the meaning of "that" (as well as "that ... not"; I know, it's terribly confusing!).

For example, "I do not doubt but he will come" means "I do not doubt that he will come", not "I do not doubt that he will not come".

I'm guessing that this is the meaning here, but I am not 100% sure.
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Thanks Mister Micawber !!

What lives but must not then be one with you?

What I don’t understand in this sentence is that when “but” means “that ~~ not”, why “not” is added here…

What lives but must then be one with you?

This one seems a lot more logical and seems to make sense….
 Mr Wordy's reply was promoted to an answer.
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Thanks Mr Wordy....

Really helping answer, indeed.....