My theological understanding, from from the beginning, of the issue of what Jesus meant when describing the Kingdom in Lk. 17.21, has been to take His phrase as referring to Himself as King in their midst. I no longer believe this view is viable.
Some commentators make much of the fact that Jesus was being questioned by the Pharisees, and therefore, He cannot refer to them as having the kingdom of God inside themselves. This is fairly misguided in my view since it seems to be a very natural question without hint of rebuke or very mild rebuke.
Of all the groups of Jews in the first century, Jesus was most like a Pharisee in doctrine and, at first, associated somewhat with them. Nicodemus and Joseph of Arimathea were probably Pharisees who later became Christians. The Apostle Paul still claimed he was a Pharisee after three missionary journeys as a Christian standing before the Council of Jews when defending the resurrection. Instead, here in Luke, Jesus told the Pharisees the reality of the New Covenant where the redeemed will know God personally through the indwelling Holy Spirit.
The primary reason for my interpretation of “within” is, if taken plainly, the text emphasizes that the Kingdom can’t be seen. Therefore, it cannot refer to Jesus as King or to the disciples as a kingdom because they were apparent. By saying nearly the same thing twice in His response, there seems to be a confirmation of imperceptibility of either its members or its physical appearance:
Being asked by the Pharisees when the kingdom of God would come, he 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.” (NET)
What did the producers of the NET bible (translators and interpreters) think when they rendered entos as “in the midst of you?” It almost seems as if they were straddling a fence by referring to the two competing ways this verse is taken. The term can really only refer to one view. The meaning is either the Kingdom is within you or the Kingdom is among you; not both. Since Jesus clearly is saying that the Kingdom is not apparent, the first view of the Kingdom inside a person is the correct view.
Finally, entos is only used twice in the N.T. with its other usage in Mt. 23.26 where it refers to the interior of a bowl or cup. Jesus, in this passage, tells His opponents to clean the inside (entos) of the cup and dish to be clean outwardly. This is where we can see a clear usage of the term to mean “inside.” Since Lk. 17.21 is the indeterminate usage, its helpful to see how it is used in its plain sense in Mt. 23.26 to gauge it for the occurrence in question.
The Kingdom is really inside Christians in the sense that they have surrendered themselves to Christ and He now indwells them. Of course Christians still pray: Your Kingdom come, where, in this instance, the external glorious reign of Christ is referred. However, now, Christians are members of that kingdom, since, as citizens and ambassadors we further it.