Mark 1:4 Commentary: John the Baptizer, the Prophesied Wilderness Prophet
John the Baptizer appears in the wilderness as the prophesied messenger of the LORD who would prepare His way (cf. Mk 1:2-3) — a way for a new exodus, a new divine encounter, a new deliverance via the LORD the King.
This preparation for the way of the Lord Jesus entails the proclamation of a preparatory baptism for those who had turned away from sin and turned to God (i.e., repentance) for the purpose of the forgiveness of sins (by God). This forgiveness will ultimately be made possible through the work of Christ on the cross.