I don't know the range of meanings and uses of the expression "the world of Humanity" as it is used in this belief system, but i think I would keep the "in."
The upper-case letter on "Humanity" but not on "world" has to be decided on the basis of the meanings and uses of this phrase. I would ordinarily think that either both should be capitalized (a distinct metaphorical place: the World of Humanity) or that neither should be (the world of humanity in its generic sense.) But don't capitalize "Humanity" as if it were a proper noun.
Don't use "feel" as a synonym for "opine."
"for his living" does not capture the meaning, as far as I can tell. "good enough to live on" is probably closer, but even so, the rest of the sentence supports this meaning without its needing to be said.
Don't use the locution "goes and does" something. Just say, "He does it."
To reverse the meaning, use "but," not "and"
Whether or not "with him" should be used depends on the meaning. Including "with him" suggests a person who is moving about. It gives the idea of "what he carries around." Since I think it is not likely that this is the intended meaning, I think the phrase could be deleted.
A person whose life condition is in the world of humanity has few wants and thinks that what he has is good enough. He never pursues what is more than necessary, but appreciates what he has.
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