Donatism is an idea from very old times. It was a way of thinking about how to be a Christian. The basic idea was that only true Christians, or people who believed the right things about Jesus and God, should be allowed to hold positions like priests or church leaders. People who weren’t “good enough” or believed differently were not allowed. It was kind of like a club that only let in people who believed the right things. Donatism was popular in some parts of the world in the early days of Christianity, but today it is not practiced much.