John the Baptist was baptizing in the wilderness and preaching a baptism of repentance for the forgiveness of sins.
This verse presents the main substance of John the Baptist’s ministry. He was not preaching an outward rite or an empty religious form, but was calling people to true repentance—a change of mind, heart, and life. His baptism was a sign that a person has recognized his sin and desires to return to God. The phrase “for the forgiveness of sins” shows that repentance brings a person to the place where, with a humble heart, he pleads for God’s pardon. John was not yet giving the full grace that would be given through Jesus Christ, but he was preparing people’s hearts to receive the true Savior.