system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Historically, why have kings/queens allowed the church to also have so much power over the people? Do they gain something from each other, or are kings/queens just scared of going to hell?
gpt
There may be something here that some people have missed.

In order to provide legitimacy to a government that not only says that this one person is in control but said control should naturally be inherited by their lineage, you need some kind of divine reasoning about how a particular family or group is destined or eligible to rule. It wasn't just that religion and the monarchy thrived off one another - they needed one another. 

The church, in times of national crisis, could place their power behind a particular family or leader and transform (typically) him from a mere warlord to ruler. By legitimizing the feudal government, the people were powerless to fight back lest they were going against god. Furthermore, other individuals who wished to seize the throne would suddenly find themselves being attacked and deposed of their lands by their allies simply because the church said that they were pretenders or upstarts to God's order. In return for that legitimacy, the feudal governments typically gave generously to the church. 

That said, it didn't hurt that those in the feudal systems tended to be quite religious. It was a system that fed itself right up until the enlightenment when democratic systems began to grow as monotheistic religious organizations began to wane in power.