Perhaps one of the more frustrating aspects of the modern western church is the lack of clarity on what the "kingdom of God" really is. I think much of it comes from emphasizing, and developing an entire narrative based on one relatively obscure statement recorded by Luke.

Once, on being asked by the Pharisees when the kingdom of God would come, Jesus replied, “The coming of the kingdom of God is not something that can be observed, nor will people say, ‘Here it is,’ or ‘There it is,’ because the kingdom of God is in your midst.”

Luke 17:20-21

Note that, instead of "in your midst", some translations will use the words "among you" or "within you". Building the entire kingdom of God narrative around the words "within you" leads to so much misunderstanding.

While it is true that there is a spiritual component to the kingdom of God. One must repent and accept Jesus' sacrifice on their behalf before you can see, or enter, or experience the blessings of the kingdom of God (John 3:3). However, this kingdom is not just a spiritual kingdom. In its fulfillment, it will be VERY REAL!!

Note that Jesus used the word "coming kingdom of God" in Luke 17:20. I believe that "in your midst" or "among you" is probably a better translation - referencing Himself as the source of that soon coming kingdom. He was addressing the Pharisees here. I doubt that His intention was to suggest that the kingdom of God was "within" them.

Moments later, Jesus turns to His disciples and states the following:

The time is coming when you will long to see one of the days of the Son of Man, but you will not see it. People will tell you, ‘There he is!’ or ‘Here he is!’ Do not go running off after them. For the Son of Man in his day will be like the lightning, which flashes and lights up the sky from one end to the other.

Luke 17:22-24

In other words, the time is coming when His disciples will long to "see one of the days of the Son of Man" - or His kingdom. He then says that if people tell them "here he is" or "there he is", don't believe them. Why? Because when the Son of Man comes, it will be "like lightning which flashes from one end of the sky to the other". In other words EVERYONE will see it. This IS the explanation that is consistent with the rest of the body of Scripture.

Immediately after the distress of those days...

Then will appear the sign of the Son of Man in heaven. And then all the peoples of the earth will mourn when they see the Son of Man coming on the clouds of heaven, with power and great glory.

Matt 24:29-30

These verses are not, by any means, the only verses that describe a very literal appearing of the Son of Man at His second coming. Before I get to that, I want to state one very important rule of interpretation:

Some passages of Scripture are more difficult to interpret correctly than others. When confronting one of these, it’s best to locate the clearest verses on the subject and use them to help interpret the difficult one.

So, to  get a more clear picture of what the kingdom of God entails, it is helpful to return to the prophets. They are not the only source. But for the sake of simplicity, it is a good exercise to look at what they said regarding the kingdom of God and compare those statements with what is said in the New Testament books.

A very simple exercise I might suggest would be to read the epilogue (final chapter) of EVERY Old Testament prophet (with a few exceptions - Jeremiah, Ezekiel, Jonah, Nahum) and then read the final few chapters of the Revelation to John - and then compare notes.

In the case of Jeremiah and Ezekiel, the kingdom verses are extensive and dispersed throughout each book. The books of Jonah and Nahum deal primarily with Nineveh. For the balance of the prophets, in this last chapter exercise, you will find extensive information about what the kingdom of God entails. As you progress through these chapters, refer forward to the final chapters of the Revelation to John (chapters 19 - 22).

The consistency is amazing!

“For just as the new heavens and the new earth,
     which I will make, will endure before Me,”
     declares the LORD,

“so your descendants and your name will endure..."

Isaiah 66:22

Then I saw a new heaven and a new earth, for the first heaven and earth had passed away, and the sea was no more. I saw the holy city, the new Jerusalem, coming down out of heaven from God, prepared as a bride adorned for her husband.

And I heard a loud voice from the throne saying:

“Behold, the dwelling place of God is with man,
     and He will dwell with them.
They will be His people,
     and God Himself will be with them as their God.

Rev 21:1-3