Luke 17

Now when He was asked by the Pharisees when the kingdom of God would come, He answered them and said, “The kingdom of God does not come with observation; nor will they say, ‘See here!’ or ‘See there!’ For indeed, the kingdom of God is within you.” vv.20-21

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Some use this verse to say that the kingdom of God is something inside believers only, yet that can't be true as He was talking to the Pharisees here who didn't believe He was God yet tells them the kingdom of God is within them. Some think that it's up to believers to bring the kingdom, yet even the Pharisees know that it is something that only God can bring when they ask when, knowing that they can't. Some will even say the kingdom of God is a "spiritual kingdom."  Adding the oblivious word spiritual to define something that they aren't able to but no one should dare second guess anything called spiritual (would you also call the pharisees spiritual since that is who Jesus is talking to here?).  Yet even using this to define it here would make other passages of scripture nonsensical when talking about the Kingdom of God.  
So what did He mean then when He said 'the kingdom of God is within you'?

Remember, the Lord has just told us that the kingdom of God does not come with observation. You cannot see it coming or map its progress. Now, He tells us why that is. The reason is that the kingdom of God does not come upon men from the outside. That is the way all human governments must come in. Yet the kingdom of God is not this way. When the kingdom of God comes, no man will watch its progress as it moves from one part of the globe to another. Instead, it comes on the scene suddenly, and appears inside of men. No one can observe its coming, for it comes within those it makes its subjects.
Moreover, the kingdom of God when it comes does not act upon men from without. In order for a human government to impose its rules upon you, it must catch you violating them (or suspect that you violated them, depending on the system of government,) capture you, and punish you. The kingdom of God does not need to act this way, however. The kingdom of God is within men, (yes, even men like the Pharisees,) and when someone violates its laws, it can act within him to bring about his punishment. Moreover, it can internally guide men into a knowledge of what exactly its rules are. Isaiah 30:21 says: Your ears shall hear a word behind you, saying, “This is the way, walk in it,” Whenever you turn to the right hand Or whenever you turn to the left.Thus the kingdom guides men from within, not by rules imposed upon them from without. The Kingdom of God is not really made up of things like palaces or ornamental chairs called thrones or other, material things that make up some governments. There is no place in the Kingdom of God that one could point to like Washington, D.C. and say, “Look, this is the Kingdom.” Instead, the Kingdom of God is all-present and all-pervasive. It reaches from the most elegant city to the crudest jungle village, from the highest official to the lowest citizen. Everywhere upon the earth the Kingdom will be actively and livingly present. There is not a dark corner or a hidden recess anywhere in the world where one can sin without the knowledge of the Kingdom of God, and no place to hide from its inevitable and total rule. For its punishments are not ones that take place outside the body, like prisons or fines, but rather sickness within the body itself. Likewise its directives are not those written on paper and stored in a library somewhere, but rather the voice of God Himself speaking to the heart of every citizen of the Kingdom directly through His Spirit. (Bible Teacher Nathan Johnson)