The New Testament tells several stories about people being baptized, but the stories do not spell out the details of how the baptism was accomplished. The apostolic writers thought that the method and procedure of baptism was so well known that they felt no compulsion to record any of the details of the ritual. Because the apostles were all Jews, they considered baptism to be a basic part of daily life which required no description.
Baptism was originally a Levitical purification rite. Most purification ceremonies, such as the purification after leprosy, require immersion into a mikvah. The Greek New Testament expresses immersion into the mikvah with the term baptizo (βαπτίζω), the word from which we derive the English term baptism. Leviticus 15 prescribes baptism as the mode of purification for a variety of ritual contaminations.
Baptism means different things to different forms of Christianity. Disagreements about the mode and meaning of baptism can be blamed, in part, on the New Testament’s scanty descriptions of the ritual. The apostles say very little about the mode, never explaining exactly how a person is to be baptized. They say a bit more about the symbolism, but they leave most of that as if it is already taken for granted.
A person needs to be ritually pure before he or she can enter the Sanctuary or eat of the sacrifices. At a minimum, purification from ritual uncleanness required a full-body immersion into mayim chayim (מים חיים): Living water, that is water collected from a natural source like a spring, a river, or rainwater, but not drawn from a cistern or well. A pool of living water is called a mikvah.
A person undergoing immersion descends into the mikvah (or river, or lake, or ocean, or whatever the case may be). The person immerses himself or herself by wading into chest-deep water and bending the knees to completely submerge himself or herself. The dunking is repeated two more times for a total of three consecutive dunks. A person who immersed himself in this manner washed away his ritual uncleanness.
All worshipers going up to the Temple underwent immersion before entering the holy place. Archaeology has unearthed the remains of many apostolic-era immersion facilities near the entrance to the Temple. These are the same immersion baths that Yeshua and His disciples used as they went up to the Temple when in Jerusalem. Archaeologists have found remains of apostolic-era immersion baths all over the land of Israel, and they consider the presence of a mikvah in an excavation as key evidence of a Jewish population.
All of this indicates that baptism was not a Christian invention or even an apostolic innovation. From the days of Moses, Jews regularly practiced ritual immersion. Anyone who became ritually unclean needed to undergo a baptism before he or she could enter the Temple or eat from the sacrifices. The priests immersed every day. After a woman completed her monthly cycle, she needed to immerse herself before she could rejoin her husband. Some pious Pharisees went beyond the letter of the law and attempted to maintain a constant state of ritual purity which necessitated regular, daily immersion.
The immersion ritual symbolizes death and resurrection. When a proselyte converts to Judaism to become legally Jewish, he passes through an immersion in the mikvah. His legal identity as a Gentile dies in the water of the mikvah, and the proselyte emerges from the mikvah reborn as a Jew. Likewise, John the Immerser employed immersion as the physical token of repentance. The penitent entering the water of the Jordan died to sin and emerged from the water reborn to a life of repentance and righteousness. Paul attached similar symbolism to the immersion in Messiah:
Do you not know that all of us who have been baptized into Messiah Yeshua have been baptized into His death? … Our old self was crucified with Him, in order that our body of sin might be done away with, so that we would no longer be slaves to sin; for he who has died is freed from sin. (Romans 6:3-7)