Matthew's reference to the kingdom of heaven was by no means purely about the future. The Lord's Prayer asks that God's will be done on earth "as it is in heaven." Add to that the fact that the very use of the term kingdom (basileia), reign or empire, was itself subversive. When one was under the control of Rome, there was only one empire and it wasn't the empire of heaven! So when Matthew's Jesus says, "Repent for the kingdom of heaven is at hand," "kingdom of heaven" is an umbrella concept for the entire mission of Jesus as he both announces and actualizes the reality of God's reign. The kingdom of heaven is about the here and the now. It's about God's subversion of ruling powers in our world and life through the ministry of Jesus and his disciples.
Mark is usually the gospel that uses the word "immediately!" to convey the sense of urgency that characterizes his gospel. But here Matthew reports that when called, the first four disciples—Simon who was called Peter, Andrew and James and John, the sons of Zebedee—"immediately" followed.
This opening of Jesus' ministry is Matthew's portrayal of how disciples should answer Jesus' call, in his time and in ours. There is no provision for taking a seat in the back row in case you want to leave early. Because there is no place to go but to follow Christ. And there is no other way to go there besides repenting, and committing oneself to a risky ministry of teaching and healing in the name of Jesus, in the here and the now, so that "The people who sat in darkness will see a great light." As you sit in the back row, in the darkened auditorium, the teacher is inviting you to take a seat closer to the front. The right answer is to get up and move forward.
O. Wesley Allen, Matthew: Fortress Biblical Preaching Commentaries (Minneapolis: Fortress Press, 2013).
Donald Senior, Matthew, The Abingdon New Testament Commentary (Nashville: Abingdon Press, 1998).