"The kingdom of God (is) righteousness and peace and joy in the Holy Spirit." The end-time in which we live is the age of the outpouring of the Spirit. Ever since Pentecost, a decisive battle has been joined between "the flesh" and the Spirit.
Only a pure soul can boldly say: "Thy kingdom come." One who has heard Paul say, "Let not sin therefore reign in your mortal bodies," and has purified himself in action, thought and word will say to God: "Thy kingdom come!"
Source: Catechism of the Catholic Church (Vatican.va)