Which sentence from king's “i have a dream” speech the reader understand the meaning of the phrase “soul force” as it is used in paragraph 8?

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“we must not allow our creative protest to degenerate into physical violence.”
“…many of our white brothers, as evidenced by their presence here today, have come to realize that their destiny is tied up with our destiny.”
“…we will not be satisfied until justice rolls down like waters and righteousness like a mighty stream.”
“we will be able to work together, to pray together, to struggle together, to go to jail together, to stand up for freedom together…”