The Protestant Reformation was a time when some people in Europe about 500 years ago didn't like some things the church was doing. The church was lead by a man called the Pope, and people thought he was doing things wrong.
One thing that people didn't like was that the church was selling some special pieces of paper called "indulgences." These pieces of paper were supposed to forgive people for their sins, but some people thought it was wrong to do that.
A man named Martin Luther was one of these people. He thought that the church was wrong, and it needed to change.
So, he wrote a list of things he thought were wrong, and he put them on the door of a church in Germany. This is called "posting" them.
People read "The 95 Theses" and agreed with what Martin Luther was saying. They started to talk about it and tell other people.
Some people who agreed with Martin Luther decided to start their own church. They called this church the Protestant Church.
The Protestant Reformation happened because some people didn't like what the church was doing. They wanted to start their own church where they could do things differently.