The less one actually knew about the genestealers, the better. They were creepy. Their first generations were almost indistinguishable from baseline humans. It was only in their grandchildren and great grandchildren that they were unrecognizable horrors. A normal person could go to school, date, even marry...a filthy xenos and never even know.

"The Saints derive their power from devotion to the Emperor. It is earned through acts and suffering." That is the nice, correct answer. One day Gadriel himself might earn the direct intercession of the Emperor.