The hostile Pharisees came to Lord Jesus Christ and demanded to demonstrate the Kingdom of God or shut up speaking about the Kingdom of God. Lord answered them: “The kingdom of God is not coming in ways that can be observed, nor will they say, ‘Look, here it is!’ or ‘There!’ for behold, the kingdom of God is in the midst of you.” (Luke 17:20-21) There are many who think God or reality, or truth is within you. Sadly, they even quote this bible verse. There are many techniques they propagate like yoga or transcendental meditation to attain that.
Observation: Lord Jesus taught that the Kingdom of God does not come by observation, which means hostile examination. Pharisees were trying to do that.
No magic: the Kingdom of God is not magic that someone can show here or there.
In your midst: Lord said that the King of Kings, referring to Himself is here, hence the Kingdom of God is in your midst. There cannot be a kingdom without a king. The unchanging Eternal King reigns over the unshakeable Kingdom. (Hebrews 12:28)
Within humans? Many misunderstand and think that God could be found within oneself and so is his kingdom. First, they misunderstand that humans are perfect. As Adam and Eve sinned, all their descendants are sinners. Unless sinners repent and invite Lord Jesus Christ, there is no hope for a sinner. Second, human beings in their pride create idols and assign them divine status. Human hand-made gods dominate religions around the world. If man could give divinity to material things, humans also could become a god. Third, many think it is not good or evil. Good and evil are myths. There is only one reality, and that is god in all, including oneself. Hence, there is no sin, so no sinners. Therefore, they are unable to repent and receive Lord Jesus Christ.
Knock at the door: Lord Jesus knocks at the door of humans. Those who listen and invite him, the triune God will indwell in them. (Revelation 3:20)
Have I invited Lord Jesus and the citizens of His Kingdom?