Below is my correspondence with a brother priest regarding the parish mission statement of the parish of which he is the pastor for just over a year, and which I recently visited.
There is another related point and it is that according to Ratzinger this expression "kingdom" in the kingdom of God means God's presence among us, the divine Visitation, God's reign. It has an active sense: God's work in the sense of God's active presence among us. God's kingdom is not a place but a Person, Three Persons.
Taken together the sense is that God is the protagonist. His kingdom is His doing through and through, and we must enter into that work, God's work, opus Dei. The other, though a very common expression in the Church, sounds Pelagian.
Joseph Ratzinger, in Chapter 3 of Jesus of Nazareth (Part One, Ignatius 2007), "The Gospel of the Kingdom of God," explains the proper understanding of the kingdom of God.
"Our main criticism of the secular-utopian idea of the Kingdom has been that it pushes God off the stage..." 55
"When Jesus speaks of the Kingdom of God, he is simply proclaiming God, and proclaiming him to be the living God, who is able to act concretely in the world and in history and is even now so acting. He is telling us: 'God exists' and 'God is really God,' which means that he holds in his hands the threads of the world. In this sense, Jesus' message is very simply and thoroughly God-centered. The new and totally specific thing about his message is that he is telling us: God is acting now--this is the hour when God is showing himself in history as its Lord, as the living God, in a way that goes beyond anything seen before. 'Kingdom of God' is therefore an inadequate translation. It would be better to speak of God's being-Lord, of his lordship." 55-56
"Jesus himself is the Kingdom; the Kingdom is not a thing, it is not a geographical dominion like world kingdoms. It is a person; it is he." 49
"Ethics is not denied; it is freed from the constraints of moralism and set in the context of a relationship of love--of relationship to God. And that is how it comes truly into its own." 62