Matthew 21:28-32 "What do you think? A man had two sons. And he went to the first and said, 'Son, go and work in the vineyard today.' 29 And he answered, 'I will not,' but afterward he changed his mind and went. 30 And he went to the other son and said the same. And he answered, 'I go, sir,' but did not go. 31 Which of the two did the will of his father?" They said, "The first." Jesus said to them, "Truly, I say to you, the tax collectors and the prostitutes go into the kingdom of God before you. 32 For John came to you in the way of righteousness, and you did not believe him, but the tax collectors and the prostitutes believed him. And even when you saw it, you did not afterward change your minds and believe him.
This parable is a very clear description of repentance. The religious leaders were, once again, questioning Him. The one son was hypocritical, acting as though he was willing to do the will of his father, but didn't. The other was disobedient, honest in his reaction by saying "I will not", but disobedient. This disobedience is all of us - we are all disobedient to God, not just prostitutes and criminals. The disobedient son changed his mind, and changed his way. Our salvation is not marked by our past, what we have done or not done; it is marked by our present; are we walking in obedience to God after having changed our mind about the severity and consequence of our sin? That is what it means to repent; when your mind changes, everything changes.
Matthew 3:1-2 In those days John the Baptist came preaching in the wilderness of Judea, 2 "Repent, for the kingdom of heaven is at hand."
Mark 1:14-15 Now after John was arrested, Jesus came into Galilee, proclaiming the gospel of God, 15 and saying, "The time is fulfilled, and the kingdom of God is at hand; repent and believe in the gospel."
What does it mean to believe?