When God made his contract (covenant or testament) with Israel, He made it with the whole nation. All the people of the nation were to be His and He was to be there God.
The Kingdom of God then was a kingdom of this world. It was the kingdom of Israel. One particular nation chosen by God to be His very own possession.
Within Israel there were many people touched by the Spirit of God, knowing Him personally as their God. There were also within Israel many who did not know Him personally but lived in persistent rejection of Him. The history of Israel and its earthly kings was a history of rebellion and sinfulness of the people of God. It was a history of their rejection of Him and His rejection of them. Yet within this national apostasy there was always a faithful remnant who, touched by His spirit, were obedient to His covenant word.
The prophets, looked forward to the time when all God’s people would know Him. When the members of kingdom of God would be identical with the people of God. When all would be forgiven of their sins and touched by the Spirit of God would know God and be known by Him.
So Jeremiah spoke of the new covenant (Jeremiah 31). And Joel spoke of the Spirit of prophecy coming on all God’s people (Joel 2). And Isaiah writes of all being taught by God himself (Isaiah 54)
The Kingdom of God that Jesus brought to the world was just such a kingdom. It was not a kingdom of this world. It was not a national Kingdom. It was not a nation like Israel. Nor was it a restored nation of Israel. It was the heavenly kingdom of the age to come, which was commenced in the death and resurrection of Jesus.
We must not confuse this Kingdom of God (or Kingdom of Heaven) with the Church. The Church is not a replacement kingdom of Israel. The Church is the gathering of Kingdom dwellers. Gathering in heaven around Christ. Gathering here on earth as we await our physical resurrection.
The Gospel we preach is the gospel of the Kingdom. We are citizens of that Kingdom and partners in that gospel. We share together in the common concern of the worldwide proclamation of our King and his Kingdom. We are sent by Him to call people into His kingdom – a kingdom that offers forgiveness to all rebels – a kingdom where every citizen knows the King personally.
All citizens of Heaven are members of the Church of Jesus Christ and partners in the Gospel of our Risen King. It is not possible to have one of these without the other two. For unlike the Old Covenant – nobody is born into this kingdom but all who are members are reborn by the Spirit of God into the kingdom of heaven.