What is the "kingdom of God" that God made available in Jesus Christ? It is a substantial (ineffable), universal, spiritual and eternal (unbegining and unending) kingdom. Kingdom means "reign, rule and authority" though many have taken it to mean something territorial as a realm with borders. I don't shackle the Kingdom with limitations that spell out a specific location with residential and physical presence. As a spiritual kingdom, it is found in the hearts of those you look to the Lord Jesus as the authority guiding and directing their lives, moreover those who have permitted the Holy Spirit to mingle with their innermost being. By saying that it is not a visible kingdom of outward observation we do not diminish its substance, because physicality / materialism is so far below God. Creation, itself, is as nothing next to the substance of God's being - Spirit.
"Now having been questioned by the Pharisees as to when the kingdom of God was coming, He answered them and said, 'The kingdom of God is not coming with signs to be observed; nor will they say, 'Look, here it is!' or 'There it is!' For behold, the kingdom of God is within you." (Luke 17:20,21)
"the kingdom of God is not eating and drinking, but righteousness and peace and joy in the Holy Spirit." (Romans 14:17)
"My kingdom is not of this world. If My kingdom were of this world, then My servants would be fighting, that I might not be delivered up to the Jews; but as it is, My kingdom is not of this realm." (John 18:36)
(People have tried to interpret Jesus' words as meaning "not of this world-system," but failed to account for the context.)
"Jesus answered and said to him, 'Truly, truly, I say to you, unless one is born again, he cannot see the kingdom of God.' .... "Jesus answered, 'Truly, truly, I say to you, unless one is born of water and the Spirit, he cannot enter into the kingdom of God.'" (John 3:3,5)
"Truly I say to you, unless you are converted and become like children, you shall not enter the kingdom of heaven." (Matthew 18:3)
"Jesus therefore perceiving that they were intending to come and take Him by force, to make Him king, withdrew again to the mountain by Himself alone." (John 6:15)
Today, many we have a new batch of people that still want to impose that upon Him in their ignorance. Their notions imply that Jesus Christ offered a worldly kingdom to the Jews, but they rejected it, so He postponed what he came for, and dying on the Cross, established a parenthetical mystery kingdom until His second Coming. When the Lord comes back, their expectation is the notion that He will accomplish what He failed to do the first time, that is to become the Priest-King in the physical Jerusalem on earth.
I have seen this called "pre-millennialism." I, personally, cannot find biblical support for the expectation of a physical kingdom here on earth. In fact, such an expectation seems to be contrary to everything the Scripture says about the Kingdom. The revelation of God's Kingdom is not limited in duration to a thousand years, but is eternal; not physical, but spiritual; not Jewish, but universal.
Neither did Jesus come to re-establish the old Davidic kingdom and thereby bring "good government" to the world. I reckon it shear folly to suggest that it is in the mind of God to reform an earthly society. You might recognize these themes in what is called "post-millennialism." The premises of both pre-millennialism and post-millennialism blind Christian believers to the present spiritual reality of the kingdom of God. Both miss the point of Jesus' statement, "My kingdom is not of this world." this physical world - (John 18:36). To me, these notions are a stumbling block, not allowing Jesus to reign in their lives in godliness right now. They are waiting for it to come upon them, rather then letting it be revealed within them.
The kingdom, or authority, or rule of God is a spiritual reality in this present age (more real then anything you can see with your physical eyes,) and it will appear in its final, perfected form when eternity is not longer masked behind this temporal cosmos, because it (time and creation) will have passed away. The kingdom is revealed here by Christ at His first advent, but only to those with eyes to see, spiritually because nobody has seen it with physical eyes at any time, nor could they. When He comes again, the whole world will have revealed the power and glory of an always current / present spiritual and eternal kingdom.
Faith receives the "already" sense of the kingdom and the "not yet revealed" sense of the same eternal kingdom as we await the final day the destruction of the temporal veil concealing the consuming fire of the Holy One.
Jesus is King. He is King "after the order of Melchisedec...King of Righteousness... King of Peace." (Hebrews 6:20-7:2) "His throne is forever." (Hebrews 1:8) He is "King of Kings and Lord of Lords" (Revelation 17:14).
We are a "royal priesthood, holy nation" (I Peter 2:9; Rev. 1:6; 5:10). Christians have been transferred into the kingdom of Christ:
"For He delivered us from the domain of darkness, and transferred us to the kingdom of His beloved Son, in whom we have redemption, the forgiveness of sins." (Colossians 1:13,14)
My commandment to you is to love those who have wronged you and forgive them, starting with your own self; and be patient with those whom you have wronged. ~ Jesus [Letters from God and His Christ - Volume 6 - Bear Fruit According to the Spirit of Truth]