Houjutsu(法術) and Houryoku (法力) "Hou" (法) means "law, principle, method" (Like, in the principles followed by a religion). Jutsu (術) means skill, craft, or in this case, magic.
So the translation for Houjutsu would be something like "religious magic". Ryoku (力) means ability. So Houryoku means "religious strength". (the power to use such magic)
What is it and where do you get it? Edit
Houjutsu is the magic that humans get from God, which is used to fight Majutsu (demon magic) .
Houjutsu can be learned by humans, so their power depends on how much they study the art. Humans can also use Houseki (法石) , or "religious stones" which weaken the Mazoku.
So religious power (Houryoku) can be acquired, but not demon power (Maryoku). Maryoku is the magical ability that the Mazoku have and it is a trait of the soul. It cannot be learned (3rd novel). So, how strong or weak your demon powers are depend on your soul and you can't improve your skills. (Sometimes though, some of your powers might be hidden).