The Kingdom of God, as taught by Jesus, is heralded by the early parables (Mark 2:21-22) that He used that seems to imply a “change of guard” – the Abrahamic covenant has now become the old religious order, and the kingdom of God has now arrived as a new covenant.
It seems that the phrase “Kingdom of God” and “Kingdom of Heaven” are used interchangeably. Specifically, the phrase “kingdom of God” occurs 68 times in ten different New Testament books, whereas “kingdom of heaven” occurs 32 times, and interestingly only in the Gospel of Matthew. One theory claims that the reason Matthew, being a Jew himself, used the phrase “Kingdom of Heaven” extensively is because Matthew’s writing is specifically targeted for Jewish readers, with the focus to show that Jesus Christ is the fulfillment of the many Old Testament’s prophecies. Devout Jews avoided using the name YHWH, and rather substituted with an appropriate phrase to depict deity. Hence, Matthew often replaced “Kingdom of God” with “Kingdom of Heaven”, whereas Mark and Luke, with their focus on Greek readership, used the phrase “Kingdom of God”.
Although, a kingdom brings the connotation of having a king, which is Jesus Himself, unfortunately the Jewish’s concept of the “kingdom of God” is very much a physical kingdom - a militant concept, where they expected Jesus to stomp against the Roman empire and establish Israel as an everlasting nation. This is further supported that the Jewish people had been living under foreign forces as early as around 750 BC since the Assyrian invasion. Furthermore, it has been more than four hundred years’ lapse since the last Old Testament, Malachi, had spoken before Jesus came. The Jews had been dejected, possibly even doubted whether God would rescue them from further foreign invasion.
Jesus on the other hand, indicated that the kingdom of God that has come is not a physical one (Luke 17:20-21, John 18:36), but a spiritual kingdom. The entrance into the kingdom of God is heralded by a new birth (John 3:5) with a need for heart repentance (Matthew 3:2). Romans 14:17 further strengthen the fact that the kingdom of God that Jesus initiated is not about the external but rather the internal.