“So those who accepted his message were baptized, and that day about three thousand people were added.”
When the first evangelistic sermon was preached and the church was established, accepting the message meant being immersed. Baptism was at the center of conversion.
Immersion brings forgiveness of sin, v. 38, and adds one to the church. What’s not to like? Let every person submit to being immersed for salvation and inclusion in Christ’s people.