It is truly a mistake to think of God’s kingdom as just a place. The whole earth is the Lord’s along with all the heavens. The Bible uses the term to refer to Christ’s ruling activity, His “reigning” in people’s hearts through their faith in Him. When we pray, say: “Your kingdom come” (Luke 11:2)
When we utter that powerful prayer, we are urging on the Holy Spirit to do His wonderful work of creating and strengthening faith in more and more people’s hearts. We are also committing ourselves to that mission as His field representatives.
You could say that this particular petition is a mission prayer. We are praying that hell will be emptier and heaven will be fuller. We are praying that the Spirit will claim minds and hearts where the darkness of Satan used to lie like a poisonous night fog. We are praying that people who are born, live, and die slaves of sin and Satan will become citizens of the kingdom of the light of Jesus Christ instead.
Every material thing that we can touch with our hands will soon be taken from us as we die. The only thing we can take wit us to heaven is people. So, we need to pray for them. Help them find out how wonderful it is to belong to King Jesus.