You can read about the birth of the church on the day of Pentecost in Acts 2. The Apostle Peter preaches the gospel publicly in Jerusalem, and many people become respond asking what to do.
Peter tells them to repent and be baptized, and many do.
The passage concludes with these words,
So those who accepted his message were baptized, and that day about three thousand people were added to them. (Acts 2:41, CSB)
3,000 brand new believers were baptized. After they were baptized, Luke writes about 3,000 people were “added to them.” Added to who? The body of believers. The local church.
Baptism is how you belong to the body of Christ, the local church.