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.
“I baptize with water those who repent of their sins and turn to God. But someone is coming soon who is greater than I am—so much greater that I’m not worthy even to be his slave and carry his sandals. He will baptize you with the Holy Spirit and with fire.
All of Judea, including all the people of Jerusalem, went out to see and hear John. And when they confessed their sins, he baptized them in the Jordan River.