Explain the power of the Catholic Church in the Middle Ages and why they had such power

Religious fervor is why they had so much power. They were practically rulers of the world back then.

And it's because all they had to say was that whatever they did was god's will and the people would support it.

Like how the Merovingians said they were descended from Jesus to get power.

The Catholic Church was so powerful in the Middle Ages because it was a strictly controlled society sanctioned and probably ordained by the church that is that each person had his place in society and that place would be handed down from one generation to the next and also they controlled people's beliefs that for example the earth was the center of the universe.