Repent, for the kingdom of heaven has come near.The kingdom of heaven refers to the rule and reign of God. John is saying that God is near. God is about to establish his rule and reign. John views himself as standing as standing on the edge of time. God is about to do something new - he is about to act in decisive way.
Once God acts then all the other kingdoms because meaningless. All the rulers, powers, authorities of this world are subdued before God.
The only way respond according to John is to repent. This is more than just a mental decision. Repentance affects the way that we live. No longer live as if the rulers of this world are in charge. We live believing, completely convinced, that God rules.
Who are the rulers in our world? How do we live under their power? What does it look like to live under the power of God?