Literally it means "religious ability". It's the amount of power that humans have to use Houjutsu (religious magic). Humans get their power from God, praying and studying the arts of such magic. Their Houryoku can be increased with more practice, unlike Maryoku.
Humans can also increase their powers by using Houseki (法石) , or "religious stones". The Houseki also weaken the Mazoku.