The Second Petition
15 Q. What do we mean by the Kingdom of God?
A. By the Kingdom of God we mean a threefold spiritual Kingdom; that is, the reign of God in us, or the reign of grace; the reign of God on earth, or the Holy Catholic Church; and the reign of God in heaven, or Paradise.
16 Q. In the words: Thy Kingdom come, what do we ask with regard to grace?
A. With regard to grace we beg that God may reign in us by His sanctifying grace, by which He deigns to dwell within us as a king in his palace; and that He may keep us ever united to Himself by the virtues of faith, hope and charity, through which He reigns over our intellect, our heart and our will.
17 Q. In the words: Thy Kingdom come, what do we ask regarding the Church?
A. Regarding the Church we ask that she may be spread and propagated ever more and more throughout the world for the salvation of mankind.
18 Q. In the words: Thy Kingdom come, what do we ask regarding Heaven?
A. Regarding Heaven we beg to be one day admitted into that Paradise for which we were created and where we shall be perfectly happy.
(Tomorrow, The Lord's Prayer - The Third Petition)
Pius X, Pope St.. Catholic Catechism of Saint Pius X (1908) (pp. 46-47). Kindle Edition.