An initial note should be stated: when you compare the parable messages from Mark 4; Luke 8 and Matthew 13, you’ll see that the terms Kingdom of God and Kingdom of Heaven are used interchangeably. So is the Kingdom of Heaven future rather than “already, but not yet”?
We first need to see that there are five facets of “kingdom” in Scripture. There terms “Kingdom of God” or “Kingdom of Heaven” both refer to God’s rule, although the uses of the term “kingdom” will reveal five different facets.
The first facet is the Universal or Eternal Kingdom and describes God’s rule over all things. He is sovereign and is transcendent over time. He eternally rules over all things and universally all things are within His control.
The second facet is the Spiritual Kingdom, which includes all people, from Adam to the present, who have trusted in God’s salvation message of the Promised Seed or the Lord Jesus Christ (the same). This is the kingdom of Matt. 6:33 and John 3:3.
The third facet is the Theocratic Kingdom, which is God’s rule over the nation of Israel as revealed through Moses in the Mosaic Law. There was the mediatorial aspect from Moses through Joshua and through the judges, then the monarchial rule from Saul through the Babylonian Captivity, after which the Times of the Gentiles began.
The fourth aspect is the Messianic Kingdom, in which Messiah (Jesus) will personally rule for the Millennium. Jesus will physically rule from Jerusalem as the fulfillment of the Davidic Covenant (2 Sam. 7:12-16). This was offered to the Jews, but by Matthew 12, was rejected and Jesus turned to proclaim the parable messages to the Gentiles. The Messianic Kingdom will be re-offered to the Jews in the Tribulation and they will respond awaiting Jesus’ arrival at the Second Advent.
The fifth aspect is the Mystery Kingdom, which was initiated, because the Messianic Kingdom was rejected. This is explained in the parables of Matthew 13. The parables describe the time between the first and second arrivals of Jesus. It would be best described as “Christendom” or the Church between Jesus two comings. It is God’s rule during the King’s absence. This is not the Eternal Kingdom, because this is a precise time compared to the eternal nature of the Eternal Kingdom. The Mystery Kingdom is not the Theocratic Kingdom, because it is not limited to one nation (Israel), but is open to all nations of Jews and Gentiles. The Mystery Kingdom is not the Spiritual Kingdom, because those in this “Christendom” are those in the Church Age, whereas the Spiritual Kingdom is comprised of believers from all time. The Mystery Kingdom is not the Messianic Kingdom, because the Messianic Kingdom was the subject of much revelation in the Old Testament, in contrast to the Mystery Kingdom revealed only in the New Testament. The Mystery Kingdom would comprise the time between Acts 2 (some include from Matthew 13 and forward) and the Rapture, but would also include the time of the Tribulation, so it extends beyond the Church.
So the Kingdom of Heaven (or Kingdom of God in Mark and Luke) is already here with Christ ruling in the hearts of those who are submitted to Him. He is the King of Kings to those who humbly seek His will and pleasure. The Messianic Kingdom offered, but rejected in Matthew 12, will be re-offered and then manifested during the Millennial rule of Jesus.
*with gratefulness to Arnold Fruchtenbaum for his Kingdom breakdown.