4 And so John the Baptist appeared in the wilderness, preaching a baptism of repentance for the forgiveness of sins.

Read Mark 1:4 Using Other Translations

John did baptize in the wilderness, and preach the baptism of repentance for the remission of sins.
John appeared, baptizing in the wilderness and proclaiming a baptism of repentance for the forgiveness of sins.
This messenger was John the Baptist. He was in the wilderness and preached that people should be baptized to show that they had repented of their sins and turned to God to be forgiven.