John’s ministry was to prepare the way for the coming Messiah (Isaiah 40:3-5). He encouraged people to repent (turn from sin), be baptized (as a sign or “fruit” of that repentance), and be forgiven (as a result of their repentance). He told people to get their hearts and lives right because the true Messiah was coming! And the Messiah would separate the good from the bad: “His winnowing fork is in his hand to clean out his threshing floor and to gather the wheat into his storehouse, but the chaff he will burn up with inextinguishable fire.”
It’s important to note that as great as John was, the least in the kingdom is greater than he was (Matthew 11:11), and we can see why here. John was still living in the old economy. He points out sin, encourages people to live righteously, and believes that will be enough to escape the wrath of the coming Messiah. He believes that the Messiah is coming to set up a political and military kingdom. He doesn’t understand that no-one can be good enough, and that’s why the Messiah came. We need grace! We understand that now, this side of the resurrection. John couldn’t understand that yet.