Question

What was the theory called that Monarch's power power came from God and could not be challenged

Answer 1

The theory was called the Divine Right of Kings.
It is the theory that the Monarch's Power comes from God, rather than his subjects, and that to challenge that power is a crime.

Answer 2

It's called the Divine Right or Kings. It's based on the belief that monarchs receive their power directly from God, therefore are responsible to no one expect God.