The power of the Church expanded during the Middle Ages in Western Europe because most common people couldn't read. The only people who could read were the priests. They would read the Bible to the people who came to church because the people couldn't read it themselves. This means that no one could check what the Church was saying to see if it was really what the Bible said. The Church could tell people what do to without having to worry about being checked.
This changed when people started learning how to read again. They could listen to something the Church said and look in the Bible to see if it was really true according to their beliefs.
Hope this helps! Please let me know if I'm wrong :)
