John's mission as the Lord's messenger was to prepare people for the arrival of the Messiah (see Luke 1:76–77). He instructed them to confess their sins, seek God's forgiveness, and be baptized. Those who did this were ready to receive the message of Jesus (see Luke 7:29–30).
John was in the wilderness, likely the desert area around the Jordan River north of the Dead Sea. This shows he was the "voice" Isaiah mentioned (Mark 1:3).
preaching a baptism: The exact origin of John's baptism is uncertain. The Qumran community practiced a "baptism" when someone became a part of the community, which they repeated regularly. But John's baptism was a one-time event. Jewish baptism for those converting to Judaism is another possible source, but it is more likely that such baptisms were not practiced until after John's ministry.