The Kingdom of God is called the Kingdom of Heaven, because it of the heavens. It, by all our understanding, has always been, or, at least since the creation.
Its nature, then, is spirit.
God is spirit, and those who worship Him must worship in spirit and truth.
John 4:24
God is Spirit, and does not dwell in temples made by hands (Acts 17:24). Nor is He honored by a physical Kingdom (John 6:63).
Paul wrote,
Now I say this, brethren, that flesh and blood cannot inherit the kingdom of God; nor does the perishable inherit the imperishable.
1 Corinthians 15:50
The Kingdom of God is not of this world, or realm, Jesus said (John 18:36). While the King James version of this verse includes the final phrase of “but now is my kingdom not from hence.”, the NASB rendering of this reads “but as it is, My kingdom is not of this realm”. The rendering of the words translated “now” that some have taken to mean that while it not of this realm presently but will be at some time in the future, could just as well be translated as the NASB does, meaning “but as it is” rather than “now”.
While Luke 17:21 in some versions reads that the Kingdom of God is within us, it is true that it could also be translated as “among us”. But, the effect is the same. Whether it was within individuals in the time of Jesus and the apostles, or whether it was merely among yet unseen (John 3:3), the fact was that the Kingdom Jesus was talking about was there.
The Kingdom is indeed invisible, and can only be entered by those who are born again. Therefore, if someone who is not born again can see or enter into it, it is not the Kingdom. See John 3:3-5.
And when Jesus was entered into Capernaum, there came unto him a centurion, beseeching him, And saying, Lord, my servant lieth at home sick of the palsy, grievously tormented. And Jesus saith unto him, I will come and heal him. The centurion answered and said, Lord, I am not worthy that thou shouldest come under my roof: but speak the word only, and my servant shall be healed. For I am a man under authority, having soldiers under me: and I say to this man, Go, and he goeth; and to another, Come, and he cometh; and to my servant, Do this, and he doeth it. When Jesus heard it, he marvelled, and said to them that followed, Verily I say unto you, I have not found so great faith, no, not in Israel. And I say unto you, That many shall come from the east and west, and shall sit down with Abraham, and Isaac, and Jacob, in the kingdom of heaven. But the children of the kingdom shall be cast out into outer darkness: there shall be weeping and gnashing of teeth. And Jesus said unto the centurion, Go thy way; and as thou hast believed, so be it done unto thee. And his servant was healed in the selfsame hour.
Matthew 8:5-13
Jesus said that the Roman Centurion had more faith than in all of Israel. This man recognized the realm of authority on Jesus that he himself knew. He was a man under authority, with people under himself. He recognized the same in Jesus, by faith, without seeing or understanding the angelic.
Jesus was such a man as that soldier, both carrying the responsibility of assignment from the Father, and also with the ability to issue charges to those under himself, which means he was in command of angels. Consider His statement.
Do you think I cannot call on my Father, and he will at once put at my disposal more than twelve legions of angels?
Matthew 26:53 NIV
Had Jesus wanted, He could have had twelve legions of angels to command. With all that potential, He chose rather to stay submitted to the mission, to submit Himself to the will of the one that had authority over Himself, and become obedient.
And, this too, was the realm that this centurion operated in. Because he recognized what He was familiar with, simply in the presence of this man with greater, but similar authority, he spoke to Jesus out of the respect that He deserved, saying He, Jesus, was not worthy to come under his own roof.
Great faith recognized the Kingdom.
Just because Jesus had a Kingdom did not mean He did what He wanted. He did only what He saw the Father do. Why does the Father do so? First, because it is who He is. But, Beyond that, because Righteousness and Justice are at the root of all that He does.
The Kingdom does not mean always getting your way. As Jesus cried out the night before His passion, saying, “My Father, if it is possible, let this cup pass from Me; yet not as I will, but as You will.” (Matthew 26:39), it was not His own will that brought Him to Calvary’s hill, but it was His Father’s.
This is the nature of the Kingdom, to do the will of the Father (Matthew 7:21-22).