“…behold, the kingdom of God is in the midst of you.” Luke17:21 English Standard Version
The kingdom of God is God’s reign; his rule. It is both present and future; inaugurated with the ministry of Christ but not yet consumated. As in the title of George Eldon Ladd’s book, it presently is The Presence of the Future.
In Christ, the kingdom has come near (Mat, 12:28; Luke 21:31, etc.). Some enter it; some do not (Mark 10:23ff.). See the ISBE, s.v. Kingdom of God
As the forerunner before Christ’s ministry, John the Baptist cried, “Repent, for the kingdom of heaven is at hand!”
Jesus began his ministry with those same words, following the temptation in the wilderness (Mat. 4:17).
In Luke, in response “to the Pharisee’s question, ‘When is the kingdom of God coming?’, Jesus can therefore answer, ‘The kingdom of God is in the midst of you” (Lk. 17:20f.; not as AV ‘is within you’).”–The New International Dictionary of New Testament Theology, s.v. King.
I. Howard Marshall, in The New International Greek Testament Commentary on Luke, states that “nowhere else is the kingdom regarded as something internal….A different translation is demanded, and is not difficult to find. With a plural noun entos means ‘among, in the midst of’ …Such a meaning gives good sense. Jesus is speaking of the presence of the kingdom of God among men, possibly as something within their grasp if they will take hold of it.”