First, as to a book, I recommend "God's Righteous Kingdom - the Law's connection with the Gospel" by Walter Chantry, 1980, Banner of Truth. This is a Trinitarian, Reformed, evangelical viewpoint, as is this answer.
Q1. Where does the term the "Kingdom of God" originate? A. Daniel 2:44, where its full title can be seen as "The Kingdom of the God of Heaven".
Q2. When did it start? In the days of the beginning of the New Testament (eg Matthew 3:17, Mark 1:15).
Q3. Were the OT believers in a future Messiah in the Kingdom of God? One would have thought the answer is "Yes", but because of the verses in answer to Q2 the answer must be "No". All the OT believers were awaiting what would become the heritage of NT believers: the Kingdom did not begin to exist until King Jesus was revealed. So OT believers are sort of "in" the Kingdom, but the Kingdom proper had not yet begun.
Q4. Did OT believers have the Holy Spirit, the same as NT believers? Yes, Psalm 51:11, but there were not so many believers in OT days: salvation was rare, whereas salvation increased during the earthly ministry of our Lord, and then was poured out at Pentecost and onwards.
Q5. Where is the Kingdom of God defined? A. Only once, in Romans 14:17. "For the Kingdom of God is not meat or drink, but righteousness, peace and joy in the Holy Spirit." It is inward, not outward; spiritual not ceremonial, for the individual. A person who believes on Jesus receives the Holy Spirit and is from that moment spiritually a member of the Kingdom.
Q6. Is there a contradiction between "only believing" and "believing and making Christ as Lord"? No. True faith involves true repentance, and both are needed for salvation and both are needed to enter the Kingdom of God: "Testifying to both Jews and also to the Greeks, repentance towards God, and faith toward our Lord Jesus Christ ... but none of these things move me.. [as long as I] testify of the gospel of the grace 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." Acts 20:21-25.
The only realistic conclusion is that when Paul was preaching "repentance towards God and faith in our Lord" he was preaching about the Kingdom of God, and how it is entered. The only thing that counts is faith expressing itself through love, Galatians 5:6; faith without works is dead, James 2:17, 20.
Q7. Why then do many Sciptures only talk of the need for faith (without mentioning the need for repentance)? Faith and repentance are two sides of the same coin. Where there is true faith there will of necessity be true repentance; where there is true repentance there will of necessity be true faith. "As a bird needs two wings to fly up into the heavens so the soul needs the two wings of faith and repentance to fly up into heaven" - the pithy-puritan, Thomas Watson. True faith is shown by repentance, and true repentance is caused by faith, and must be mixed with faith else it is not acceptable. Our works play no part in deserving or earning mercy, but are a consequence of mercy already received by faith alone, by grace alone, because of Christ's finished work alone, as revealed in Scripture. Our repentance is as a child returning to a loving father, not as slave cringing, fearfully, crawling back in fear. It is confident faith in the mercy of God that draws the sinner home in grateful (amazed) repentance
Q8. Our Lord said he must preach the kingdom in other towns and villages also, for that is why he was sent. Luke 4:43. So where is this preaching of the Kingdom of God in the Gospels? A. It is everywhere!!! The Sermon on the Mount is preaching about the Kingdom.. it is spiritual preaching, it is describing those who are in the Kingdom in the Beatitudes (Matthew 5:1-15). All his preaching is 'Kingdom of God' preaching. It is about spiritual, inward, submission to God and His Word and will.
Q9. Why is the term 'Kingdom of God' so popular in the synoptic Gospels, Matt, Mark, Luke, but not so popular in John's Gospel? I might be wrong but (it makes sense to me anyways(!)) I think it is the general view that the Kingdom of God prophecied by Daniel in Daniel chapter 2 was highly anticipated and longed for by the Jews during the earthly ministry of our Lord right up to the time of the first Jewish Rebellion against Rome. It fizzled out for a while after the disastrous rebellion AD 66-70. And the synoptics were written before AD 66, John was written after AD 70.
Q10. Is there any Scriptural support for linking the Kingdom of God with the Millenium? Not as far as I know: the link is assumed... but I might be wrong. As far as I know the two are not mentioned together anywhere in the NT.
Q11. What is the chronological progress of the Kingdom?
- It is first mentioned as to the timing of its coming in Daniel chapter 2.
- It begins with the coming of the ministry of our Lord Jesus.
- It is entered by one individual at a time as they repent and believe.
- It is greatly extended at Pentecost.
- It continues to grow throughout the Gospel Age.
- It will reach full consummation with the return of our Lord Jesus to resurrect all nations, to judge the living and the dead, and to take into Glory his Church to enjoy the fullness of the Kingdom of Heaven/God forever.
Q12. What does Daniel teach us about the coming Kingdom in chapter 2?
- It will start very small but grow into a very large Kingdom.
- It will last forever.
- It will not be man-made, but of divine origin.
- It will be very different from human kingdoms in other ways as well.
Q13. How will it be different from man-made kingdoms?
Man-made kingdoms are violent, murderous, thieving. They make progress by force of arms, by sieging and starving cities into submission. Once captured some of their young men are castrated and then taken away from their parents to the conquering king's palace, or relocated whereever the king wants. Man-made kingdoms exist for the benefit of their king, who generally selfishly does not concern himself with the feelings of his subjects. They do not care how much suffering and destruction they cause. All this is well described in Daniel chapter 1: this chapter not only introduces the book, it describes what the kings of the earth are like, so when we get to chapter 2 we know what the kingdom of God will not be like. And, of course, the Kingdom of God will not be like this because its king, The King, will be the opposite of all this.
Q14. How is this Kingdom entered? No one enters the Kingdom by any force, or violence. Individuals only enter by personal choice, where God has graciously impulsed them to faith and repentance.
Q15. Where is the Kingdom of God today? It is the Church Universal, and consists of all who have truly repented and trusted in Christ for salvation. As such it is not visible to the eye, because, though we can know if we are in it, as for others we accept their testimony as long as it holds true to a Christian life of ongoing repentance and faith. God knows those who are His, we cannot often/usually know concerning others with certainty.
Q16. Where is the Kingdom of God manifested on earth? The most visible expression of the Kingdom of God is the church/the local church, despite its many possible failings, both of doctrine on non-vital issues, moral failings, and failings of practice. The more obedient and in harmony with God's word the more it manifests the Kingdom of God. Where churches have persecuted those who not have not agreed, such as Zwingli putting to death anabaptists, or Calvin encouraging the killing of a non-Trinitarian, and the Catholic church persecution of "heretics" and heretics, these have been serious failings from NT teaching, undermining/destroying that church's calling to a reflection of the Kingdom of God: the Kingdom of God is entered by personal choice in response to the preaching of the Gospel, not by force of arms.
Q17. Jesus said to the chief priests and elders of the people: "Therefore I say to you, the Kingdom of God shall be taken from you, and given to a nation bringing forth the fruits thereof" Matthew 21:43. Does this mean that the Kingdom of God existed in the OT era, and was the nation of Israel/Judaea? A. No, it was not the true Kingdom of God. It was only a picture of the true Kingdom of God. It had some of the outward form of the (coming) kingdom but not the reality. Only individuals who have the Holy Spirit, given as the gift via repentance and faith are in the Kingdom (Romans 14:17). The OT nation of Israel/Judaea was a representation of the Kingdom, but not in reality.
Q18. Is the Kingdom of God a coming kingdom when God will finally rule over the nations, such as taught by the Watchtower/Jehovah's Witnesses? It will be in Glory after Christ has come in triumph to judge the nations. Then, the Kingdom shall be a demonstration of God's infinite power over all things, in a way which will have been hidden up to that time. But now, the Kingdom is best described as a Kingdom of Grace, which God permits/brings sinners into, upon mere repentance and faith in the work and person of the Son of God, God and Man, two natures in one Person. Of course, Daniel is very clear that God rules at all times, His purposes and plans are being carried out fully both by those who want to live for Him and those who do not. Their evil deeds are part of His plan too (Genesis 50:20; Acts 2:23). God reigns at all times, fully, he is in total control at all times (Daniel 2:21, 4:3, 4:17, 4:25, 4:32-35, 5:21, 6:26-27, 7:14, 7:27 ... what could be clearer than all these verses??).
resource-request
question per your final paragraph.