Mark 1:5-6 Commentary: John the Baptist, Elijah, Jordan
John the Baptist has a popular ministry of repentance-baptism in the wilderness at the Jordan River, a location with a rich Old Testament history. The people of Jerusalem were coming out in droves, confessing their sin.
Moreover, John is dressed like a prophet — likely the prophet Elijah, who was prophesied to come before the coming of the LORD Himself (as John comes before the Lord Jesus).