The establishment of the kingdom of God began with the coming of Jesus, but it has not yet reached its fulfillment when everything shall be subject to Him. Recall that in Jesus’ temptations, Satan told Our Lord that the earthly kingdoms had been “delivered” to him. The First Letter of John says, “We know that we are of God, and the whole world is in the power of the evil one” (John 5:19).
When Jesus spoke of His Second Coming in Luke 21, He told His disciples, “when you see these things taking place, you know that the kingdom of God is near” (Luke 21:31). So in this petition of the Our Father we pray that God will bring His kingdom to fulfillment in our midst.