The Greek word for kingdom is basileia which indicates the rule of a sovereign. God's rule spans all time and is above all other kingdoms and rulerships. His dealings with man at various times throughout human history has been according to man's state and relationship to Him. In the time period following his resurrection, Jesus Christ spoke to his followers about the Kingdom of God. He spoke, as indicated above, about the coming of the holy spirit. Luke tells more about the content of Jesus Christ's speaking regarding the Kingdom of God:
And said unto them, Thus it is written, and thus it behoved Christ to suffer, and to rise from the dead the third day:
And that repentance and remission of sins should be preached in his name among all nations, beginning at Jerusalem.
And ye are witnesses of these things.
And, behold, I send the promise of my Father upon you: but tarry ye in the city of Jerusalem, until ye be endued with power from on high.
Jesus Christ spoke of his death and resurrection, remission of sins, and the promise of God which was to be endued (clothed with) power from on high. This is the substance of his teaching regarding the Kingdom of God in the time period following his resurrection.
When they therefore were come together, they asked of him, saying, Lord, wilt thou at this time restore again the kingdom to Israel?
While Jesus Christ spoke regarding the Kingdom of God, his followers asked about another kingdom. They wanted to know if he was going to restore the Kingdom to Israel. In the days of the prophet Samuel, the children of Israel had rejected God's rule over them and requested their own earthly kingdom.
1 Samuel 8:7
And the LORD said unto Samuel, Harken unto the voice of the people in all that they say unto thee: for they have not rejected thee, but they have rejected me, that I should not reign over them.
David served God as king over this earthly kingdom and sought God to the end that God called him "a man after God's own heart." God promised that the Christ would also serve as king over Israel. However, as a nation, Israel rejected Jesus Christ when he came just as they had rejected God as ruler over them. While Jesus Christ was attempting to give his followers the good news regarding restoration of a spiritual relationship with God, they were interested in restoration of an earthly, political kingdom. His answer to their question was to bring them back to the substance of his teaching regarding the Kingdom of God:
And he said unto them, It is not for you to know the times or the seasons, which the Father hath put in his own power.
But ye shall receive power, after that the Holy Ghost is come upon you: and ye shall be witnesses unto me both in Jerusalem, and in all Judaea, and in Samaria, and unto the uttermost part of the earth.
The restoration of an earthly kingdom for Israel has been set aside to be revealed at the Father's own discretion in a time to of His choosing. It is not something for us to be concerned about now. BUT - the power of the holy spirit given on the day of Pentecost is our concern. We are to receive that power and thereby witness the resurrection of the Lord Jesus Christ. This is to be the substance of our preaching regarding the Kingdom of God.
And now, behold, I know that ye all, among whom I have gone preaching the kingdom of God, shall see my face no more.
The Apostle Paul spoke to the elders and believers at Ephesus regarding the Kingdom of God. What was the substance of Paul's teaching at Ephesus? The book of Ephesians!!!
The eyes of your understanding being enlightened; that ye may know what is the hope of his calling, and what the riches of the glory of his inheritance in the saints,
And what is the exceeding greatness of his power to usward who believe, according to the working of his mighty power,
Now unto him that is able to do exceeding abundantly above all that we ask or think, according to the power that worketh in us,
Finally, my brethren, be strong in the Lord, and in the power of his might.
The Apostle Paul taught the Ephesians about the power of God which came on the day of Pentecost. He taught them about a new administration in which believers are part of one body, raised with Christ and seated at the right hand of God. He taught them "the revelation of the mystery." Therefore, when God's Word declares that Paul spoke regarding the Kingdom of God it is clear that God's dealings with believers today, as ruler of His Kingdom, is according to the revelation of the mystery and the power which He gave us on the day of Pentecost.
And Paul dwelt two whole years in his own hired house, and received all that came in unto him,
Preaching the kingdom of God, and teaching those things which concern the Lord Jesus Christ, with all confidence, no man forbidding him.
It was during this time period that Paul was given the revelation of Ephesians, Philippians, and Colossians. The greatness of our completeness in Christ and details regarding the administration of the mystery. The content of his teaching at Rome was Jesus Christ ACCORDING TO the revelation of the mystery.
Now to him that is of power to stablish you according to my gospel, and the preaching of Jesus Christ, according to the revelation of the mystery, which was kept secret since the world began,
Paul taught the Romans, Ephesians, Philippians, Colossians, and all who came unto him, that which God revealed to him regarding the Kingdom of God: the great mystery of our standing as sons and daughters of God raised with Christ and seated at God's right hand: completely completely absolutely complete in him. We suffered with Christ, died with Christ, were buried with Christ, were raised with Christ, were seated with Christ at God's right hand, and the Word of God declares that we will reign with Christ.
For the kingdom of God is not in meat and drink; but righteousness, and peace, and joy in the Holy Ghost.
1 Corinthians 4:20
For the kingdom of God is not in word, but in power.
There are many who still seek,
as did the Apostles on the day of the ascension, an earthly kingdom and
a man to rule over them. And although that man be the Lord Jesus
Christ, such desire is still the rejection of God's Kingdom and the power
which He gave on the day of Pentecost.