Once upon a time, there were many people in a big land called Slavia. They didn't all believe in the same thing, and some of them were worshipping many different gods like Perun and Veles.
One day, two guys named Constantine and Methodius came to the land of the Slavs, and they wanted to tell everyone about a new god named Jesus. They wanted to teach the people about Christianity.
Constantine and Methodius thought it would be a good idea to speak in a language the Slavs could understand, so they created a new alphabet called the Cyrillic alphabet. This helped them translate the Bible from Greek into the Slavic language.
The people of Slavia were very interested in what the two men had to say about Jesus, and they listened to them. Over time, many Slavs became Christian, and they started building churches to worship in.
The Christianization of the Slavs was a big success, and many of the Slavs eventually stopped worshipping their old gods and turned to Christianity. Today, many people in Slavic countries like Russia, Ukraine, and Belarus still follow the Christian faith.