In this part of the Our Father we echo that well known invocation Marana tha - Come, Lord Jesus. This is because when we pray for the coming of the Kingdom, we also pray for the coming of its Sovereign, Jesus.
|Fra' Angelico, Christ's Glorified in the Court of Heaven|
In another sense, according to St. John Chrysostom and St. Augustine, the “kingdom” is eternal life. We look forward to obtaining and sharing in eternal happiness in heaven. As Our Lord later says in the Gospel of Matthew, “Come, O blessed of my father inherit the kingdom prepared for you from the foundation of the world” [Mt 25:24]. In saying the words, “thy kingdom come”, therefore, we are making a statement of faith of the world to come.
Further, St. Thomas Aquinas notes, we can think of these words as asking for the Lord to reign over us: “for when we serve justice, then God reigns, but when we serve sin, the devil does: let no sin reign in your body, (Rom 6:12); they have not rejected you but me, from reigning over them (1 Sam 8:7).” So, on this understanding, we are asking the Lord to make us his agents, to reign in our heart in order that we act with charity and in accordance with the God’s law; in order that we may do justice rather than evil.
Of course, these three ways of reading the words are closely connected. If we synthesise what we have said already, we could say that we invoke God’s blessings for the present, serve justice through the practice of charity and avoidance of evil, and that we do all of this in the hope of gaining a place in heaven.
As far as summaries of the Christian life go, that’s not bad in three words! Then again, one shouldn’t be surprised - they are Jesus’s words after all!