Jewish immersion was rooted in the cleansings found in the Law (Leviticus 6,11,13-17) and the Pool of Siloam (John 9) and Pool of Bethesda (John 5:2-9) near the Temple Mount were used for this purpose. These were self-administered.
The baptisms of John and of Jesus were however not for purification but rather repentance (Mark 1:4; Luke 3:3; Matthew 3:11; Acts 13:24; Acts 19:4) and were not self-administered.
A legal discussion (zētēsis) with an individual and John's disciples arose over the difference. During this debate the baptisms that the disciples of Jesus were performing surfaced. Jesus was one or two days away from Jerusalem and John the Baptist was 6 or 7 days away. They were not on the way to the Temple, but rather pulled people away from the temple.
John appeared, baptizing in the wilderness and proclaiming a baptism of repentance for the forgiveness of sins. And all the country of Judea and all Jerusalem were going out to him and were being baptized by him in the river Jordan, confessing their sins.
No other itinerant preacher of the time, nor any other prophet of the Old Testament performed such baptisms. While Judaism later adopted baptism as an initiatory rite for proselytes, there is no pre-70 AD evidence of its use.
It is also not part of the Nazarite vow (Numbers 6:1–21) to which Jesus was not submitting when He was baptized, for Jesus drank wine (Matthew 11:18–19), touched the dead (Mark 5:41), and He cut His hair (no account of him not doing so). This assertion unfortunately is frequently heard.
So, these baptisms were innovative. All Old Testament washings needed to be repeated. The baptisms of John and Jesus were one time.
And it happened that while Apollos was at Corinth, Paul passed through the inland country and came to Ephesus. There he found some disciples. And he said to them, “Did you receive the Holy Spirit when you believed?” And they said, “No, we have not even heard that there is a Holy Spirit.” And he said, “Into what then were you baptized?” They said, “Into John's baptism.” And Paul said, “John baptized with the baptism of repentance, telling the people to believe in the one who was to come after him, that is, Jesus.” On hearing this, they were baptized in the name of the Lord Jesus. And when Paul had laid his hands on them, the Holy Spirit came on them, and they began speaking in tongues and prophesying. There were about twelve men in all.
Note please the phrases “Into John’s Baptism” and “baptized in the name of the Lord Jesus”.
Baptism is a transition from a before state to an after state. The individual ever after becomes identified by that name.
He who comes from above is above all. He who is of the earth belongs to the earth and speaks in an earthly way. He who comes from heaven is above all. He bears witness to what he has seen and heard, yet no one receives his testimony. Whoever receives his testimony sets his seal to this, that God is true.
In taking their baptism, the individual declared the testimonies of John the Baptist or Jesus Christ were true.
But more than that, in taking their baptism they are saying that God’s testimony heard by John and by Jesus is true.
If we receive the testimony of men, the testimony of God is greater, for this is the testimony of God that he has borne concerning his Son. Whoever believes in the Son of God has the testimony in himself. Whoever does not believe God has made him a liar, because he has not believed in the testimony that God has borne concerning his Son. And this is the testimony, that God gave us eternal life, and this life is in his Son. Whoever has the Son has life; whoever does not have the Son of God does not have life.
So, what statement did they hear God make?
God is true.. . . and behold, a voice from heaven said, “This is my beloved Son, with whom I am well pleased.”
He was still speaking when, behold, a bright cloud overshadowed them, and a voice from the cloud said, “This is my beloved Son, with whom I am well pleased; listen to him.”