John the Baptist preaches saying “repent for the kingdom of heaven is near.” A person who repents stops rebelling and turns around to start following god. Repentance means we must confess our sins and turn away from them. I find that this is nearly impossible on my own. But with god all things are possible. We need to stop trying to do it on our own and rely on god to help us.
John the Baptist helped prepare people to meet Jesus. As a believer I should be following in his footsteps. I cannot concert anyone but I can introduce them to Jesus and then let god do the rest.
John also confronts the religious leaders calling the a brood of vipers.
Jesus comes to John to be baptized. This may have been his sign that he was ready to take on the sins of humanity.
Matthew 3:15 NIV
 Jesus replied, “Let it be so now; it is proper for us to do this to fulfill all righteousness.” Then John consented.
Matthew 3:16-17 NIV
 As soon as Jesus was baptized, he went up out of the water. At that moment heaven was opened, and he saw the Spirit of God descending like a dove and alighting on him.  And a voice from heaven said, “This is my Son, whom I love; with him I am well pleased.”
With the numbers of people coming to John to be baptized there must have been many eye witnesses to this event.