We recently got the opportunity to go hands-on with this faction and meet the Sisters of Battle, the cleansing arm of the Inquisition whose unofficial motto is "Innocence proves nothing."
It's no surprise, then, that humanity might turn to religion to unify and focus their efforts to survive, "religion" in this case meaning a cult of personality surrounding a half-dead God-Emperor. It's filled with drooling monstrosities, rampaging xenos, alien gods that hunger to eat souls and approximately six million ways to die - many of them slow, all of them painful. The Warhammer 40,000 universe is an awful place for your average human.