What it is, where it comes from, and what the New Testament says it means.
Baptism is a ritual use of water — by immersion, pouring, or sprinkling depending on the tradition — that the New Testament presents as the standard way someone publicly identifies with Jesus and the Christian faith. The word comes from a Greek verb, baptizo, meaning “to dip” or “immerse.”
Before Jesus's public ministry, John the Baptist practiced a baptism of repentance in the Jordan River. Jesus himself is baptized by John, an event all four Gospels treat as significant — the Gospels describe the Holy Spirit descending like a dove and a voice from heaven identifying Jesus as God's Son.
After his resurrection, Jesus instructs his followers to “go and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost” — the passage most directly connecting baptism to the church's ongoing practice.
Paul explains baptism as symbolizing union with Jesus's death and resurrection — going under the water pictures being “buried” with Jesus, and coming back up pictures rising to “newness of life.”
Acts records baptism happening immediately upon someone's profession of faith, including 3,000 people in a single day at Pentecost (Acts 2:41).
Christian traditions differ on whether infants or only believing adults should be baptized, and on immersion versus other methods — one of the most long-standing points of disagreement across denominations.
Ritual water immersion for purification predates Christianity in Jewish practice, giving John's and the early church's baptism a recognizable context for its first audience.