Okay kiddo, so a long time ago there was a group of people called the Roman Empire. They were very powerful and had control over a lot of land and people. They had their own beliefs and gods that they believed in.
But then there was another group of people called the Christians. They believed in a different god and had their own beliefs. This group eventually became a pretty big deal and started growing in numbers.
This is where the clash between the church and the empire comes in. The empire didn't like the fact that there were people who didn't follow their gods and beliefs. They wanted everyone to believe in the same things they did.
But the Christians didn't want to give up their beliefs. So there were conflicts and battles between the two groups.
Eventually, the Christian church became very powerful and influential and started to gain more control. They became a sort of rival power to the Roman Empire. This made the Empire nervous and they clashed even more with the church.
But over time, the church became stronger and the Roman Empire weakened and eventually fell apart. The church became a very powerful player in the world and had a lot of influence over people's beliefs and actions.
So that's the ELI5 version of the clash between the church and the empire, kiddo. It was a long, complicated battle between two very different groups with different beliefs and values.