In the Lord's Prayer, "thy kingdom come" refers primarily to the final coming of the reign of God through Christ's return. But, far from distracting the Church from her mission in this present world, this desire commits her to it all the more strongly. Since Pentecost, the coming of that Reign is the work of the Spirit of the Lord who "complete(s) his work on earth and brings us the fullness of grace."
Source: Catechism of the Catholic Church (Vatican.va)