Okay, kiddo, so back in a long time ago, there were a group of people called the Jews who believed in God and followed certain rules and traditions. And then, this guy named Jesus came along and started talking about how God was his daddy and how people should love each other and be nice.
Some people thought Jesus was really cool and believed that he was the son of God. They started following his teachings and formed a new group called Christians. But not everyone liked Jesus and the things he was saying. Some people thought he was causing trouble and wanted him to stop.
Eventually, Jesus was arrested and killed by the authorities. But his followers didn't stop believing in him or following his teachings. They continued to spread his message and convert more people to Christianity.
This caused a rift between the Jews and the Christians. The Jews believed that Jesus was not the messiah they had been waiting for, while the Christians saw Jesus as their savior. Over time, the two groups grew further apart and developed their own separate beliefs and practices.
So, that's how early Christianity split from Judaism. It was because some people believed Jesus was the son of God and others didn't.