John the Baptist is called by God as the last prophet whose ministry of preparation heralded the coming of the Kingdom of Heaven. The King (Jesus) had arrived to forgive sins. Matthew 3:1-3 is John's call to repentance, not admission or confession of sin only. Biblical repentance is not merely a change of mind, but is more accurately a change of necessity through contrition (or sorrow). Godly sorrow or grief over sin is a work of the Holy Spirit and John the Baptist's message was to repent as Godly sorrow enables one to seek God's mercy.