"Hou" (法) means "law, principle, method" (like the principles followed by a religion). "Jutsu" (術) means "skill, craft or magic". The translation for houjutsu would be literally "religious magic". Ryoku (力) means ability, so houryoku means "religious strength" (the power to use such magic).
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, so how strong or weak your demon powers are depend on your soul and you can't improve your skills (though some of the powers might be hidden).