So church served as a machine of propaganda, to justify himself.
So a king theoretically could try to destroy the church, but it had to be done carefully, to not break the system.
So is the nobility.
There's a lot of nobility and king could abuse them within reasonable limits.
If one decided to revolt, there will be others who will keep him in place.
Each had a small army, but there were lots of them.
And they all swore allegiance to the king, also being under the influence of the church and breaking this oath would put them in hell, or at least will show them to be sinners in eyes of their people.
And there were cases when some member of nobility became strong enough to overthrow the king and become a new king.
Or when enough nobility agreed to unite to revolt, but this is quite dangerous move, so they wouldn't do that without a reason.
This is not some hypothetical , this is generally how things worked.
Of course, with some local differences.
But generally, that was the way.
When it came to not nobility, but 95% of population, which were peasants working in the field, king's power over them were absolute, legally.
They could stage a revolt, but like almost none of these were ever successful and almost always turned into a bloodbath.
They also reproduced a lot and were highly replaceable, as they were unqualified labor force.
But this changed around the start of the industrial revolution.
Guess what, technology became more complicated, whole new industries appeared, economy demanded a lot of educated people.