Okay, so there was a big group of people a long, long time ago who all believed in the same things and went to the same church, called the Church of Holy Trinity. But then, these people started to disagree about some important things, like how the church should be run and what they should believe.
Some people thought one way and some people thought another way. Eventually, the group split into two different churches, one called the Catholic Church and the other called the Orthodox Church.
Now, both of these churches still believe in God and the teachings of Jesus, but they do some things differently. They have different leaders, different ways of doing their religious ceremonies, and different rules to follow.
Basically, the Church of Holy Trinity split into two because people couldn't agree on how to do things and wanted to do them their own way.