The kingdom of God is the centerpiece of Jesus’ teachings. Matthew speaks instead of the “kingdom of heaven.” However, a number of passages in Matthew are virtually identical to those in Mark and Luke, except for the substitution of “kingdom of heaven” for “kingdom of God.” Thus, the same reality is intended. The Gospel of John mentions the kingdom only twice but refers many times to the closely related concept of eternal life.
For hundreds of years, the Jews had been expecting the decisive intervention of God to restore the glory of Israel and defeat its enemies. When John the Baptist and then Jesus proclaimed that the kingdom was at hand, it was certainly understood in terms of this expectation. (Marshall, Kingdom of God, Kingdom of Heaven)
However, the kingdom initiated by Jesus is not the earthly kingdom that was widely inferred from the Old Testament prophesies. It is a spiritual kingdom that is now growing in the hearts of men and women, and it will find its fulfillment in the eventual sovereign rule of God and defeat of all evil. Those people who choose to belong to God’s kingdom and serve Him are those who are destined to inherit eternal life in God’s presence.