Nicene Christianity is a way of believing in God that was established a long time ago by some really smart people. They got together to talk about what they believed, and they wrote down some important things about God that they thought everyone should believe too.
One of the main things they talked about was the Trinity. This is the idea that there is one God, but this God is made up of three parts: the Father, the Son (Jesus), and the Holy Spirit.
The people who talked about Nicene Christianity also believed that Jesus was both fully God and fully human. This means that Jesus was born as a human baby, just like everyone else, but he was also God at the same time. They also believed in something called "original sin," which means that everyone is born with a bad nature because of something that happened a long time ago when the first humans disobeyed God.
Nicene Christianity is still very important to many people today, and it helps them understand how they can have a closer relationship with God.