What does Mark 1:4 mean?

John 1:6–8 describes John the Baptist as the witness to the Light, which is Jesus. Luke 1:5–25 goes into greater detail about John's birth to an elderly priest and his formerly barren wife. Mark simply says, "John appeared." He introduces John as an adult already performing his public ministry—calling people to repent of their sins and baptizing those who did.

Baptism did not originate with John. According to Jewish sources, a stepped bath, called a mikveh, was ordained by the Midrash to purify people before entering the Temple, to cleanse the bride and groom before marriage, and for women after birth (Leviticus 12:1–5) or after menstruation (Leviticus 15:19–30). Its most revered use, however, was for Jewish converts. The mikveh needed to be filled (or mixed) with God-given water, as from rain, a river, or a melted glacier. Although the Jews currently have no temple to enter, the mikveh is still used for the latter purposes. In Jesus' time, it's believed it was also used to convert from one Jewish sect (Essene, Pharisee, etc.) to another.

"John's baptism," then, is a purification rite for those who agreed with his call to repent for the forgiveness of their sins. Rather than asking people to offer a sacrifice in the temple in Jerusalem, John offers a radical alternative for his era: simply repenting and being baptized. This message of repentance would be continued and furthered by Jesus.