From The Gospel of Matthew, by Curtis Mitch and Edward Sri, commenting on Matthew 3:1-12
At the heart of John’s prophetic message is the challenge to repent, which in Greek means to change one’s mind or perspective. In the Jewish tradition the Hebrew word for repent means to turn around, or return. John is calling for a complete change in thinking and conduct—a decisive, fundamental change of direction in one’s life.
In this, John stands in the tradition of the Old Testament prophets who called on Israel to turn away from sin and return to the Lord. Behind the prophet’s call to repentance is a conviction that the nation is on a path to destruction and in need of radical reorientation. But John’s call to repentance comes with an even greater sense of urgency because he announces something no prophet before him could proclaim: the kingdom of heaven is at hand. Israel faces a unique, now-or-never opportunity to repent.
The “kingdom of heaven” does not refer to a place—a place, for example, where God and the angels dwell—but to God’s dynamic activity as ruler. God alone was king over Israel, and the prophets foretold that he would come as king, establish his reign in Jerusalem, and bring justice on earth. Thus in proclaiming that “the kingdom of heaven is at hand,” John is announcing that God’s promised reign is now dawning on Israel and the world.