The ante-nicene period was a time a long, long time ago, before the people who followed Jesus became a very big group. This was before they had big fancy churches and before they had written down all of their beliefs in one big book like the Bible that we have today.
During this time, lots of different groups of people had different ideas about who Jesus was and what he wanted people to do. Some people thought he was just a regular person who taught good things, while others thought he was a really special and important person who was sent by God to save people.
Even though people didn't all agree on everything, they all believed in Jesus and wanted to follow him. They would meet together in small groups and share their ideas and stories about Jesus.
It wasn't until later, when the group of people who followed Jesus became bigger and more organized, that they wrote down all of their beliefs in one big book and decided on certain things that they all agreed were true. But during the ante-nicene period, people were still figuring out what they believed and what it meant to be a follower of Jesus.