What is defined as the "kingdom of God"?
According to Jesus, the coming of the kingdom of God cannot be observed, and no one will announce, "Look, here it is", or, "There it is." For behold, the kingdom of God is among you (Luke 17: 20-21).
So that would imply that the kingdom of God isn't something tangible. It is among us - or perhaps, within us.
The Catholic Encyclopedia explains that in the mouth of Christ the "kingdom" means not so much a goal to be attained or a place — though those meanings are by no means excluded; cf. Matthew 5:3; 11:2, etc. — it is rather a tone of mind (Luke 17:20-21). It stands for an influence which must permeate men's minds if they would be one with Him and attain to His ideals; cf. Luke 9:55.
In other words, the kingdom of God is the ruling of God in our hearts; it means those principles which separate us off from the kingdom of the world and the devil; it means the benign sway of grace; it means the Church as that Divine institution whereby we may make sure of attaining the spirit of Christ and so win that ultimate kingdom of God where He reigns without end in "the holy city, the New Jerusalem, coming down out of heaven from God" (Revelation 21:2).
So how is the kingdom of God 'among us'?
Fr. Raul C. de los Santos (in 365 days with the Lord) posed some good questions to reflect on:
Are we a poor substitute of God's warm hand?
Do we miss God's presence in our families and in moment of silence and kindness?
Do we not see God's hand in sparks of inspiration and movements of grace?
Have we been callous to God's invitation to pray and be still?
Fr. Raul also pointed out that one of the 1st lessons we learn in religion is that God is everywhere. Thus, he invites us to pray "Forgive us Lord, when we seek you elsewhere except in our hearts and in the hearts of people. Amen."