‘The kingdom of God is in the midst of you.’
Think of the timing of the authorship of Luke - some 70 years (perhaps - it is a little vague, but not before 50 years) after the resurrection. The early christians were expecting an imminent return of the Lord, this time in Glory, to bring in the end of days and the resurrection of all. In fact, Jesus, and indeed most of the people who knew him, had died, and the second coming of the messianic saving king had decidedly not happened. It is likely that Luke wrote today's Gospel to tell them, the second or even third generation christians, that they should not be looking to the return of the king, but to be looking among themselves, to see (to it) that the kingdom of God is formed by them.
There are perhaps three ways to read this "in the midst of you" or as the Jerusalem Bible had it "within you".
- 'already here' , you live in the kingdom
- 'within your grasp' - you just need other make the effort to reach out, and live in it
- 'a spiritual reality' - just live as if this was true, and you make it so.
I like the third most of all, but you are open to make your own interpretation. Just - live faithfully by your view. We are all destined for the same eternal Kingdom, that is assured to us by Christs Resurrection - but we are called to life our part, in the way we see to be true.