It's stuff from "Might is Right", look into that book.
As for your question...one could take "Poor in spirit" to mean weak willed, spineless, the kind of person who when faced with any kind of adversity will just give up in what ever they are doing or activity they are pursuing, basically everything a Satanist isn't. The ability to persevere through adversity and "hard times" is a benchmark of the Satanists personality and "spirit", IMO.
"Inferior organisms succumb and perish or are enslaved. Superior organisms survive, propagate, and possess." - Charles Darwin