commandment

If someone believes that God has directed them to do something, it's a commandment. If your parents grimly order you to clean your room, you can also consider that a commandment.

Strictly speaking, a commandment is divinely ordered, like the Ten Commandments in the Bible. But you should feel free to use this word for any solemn, serious directive or set of guiding principles. The commandments of your French club, for example, might include "no speaking in English" and "don't be late — unless you brought croissants."