Like the petition 'hallowed be thy name', this expresses a desire found only in the hearts of true Christians.
God Reigns
God already reigns over everything. He governs all his creatures and all
their actions. "Whatsoever the Lord pleased, that did he in heaven, and in
earth, in the seas, and all deep places" (Psalm 135 v6). Even the wicked
unconsciously fulfil the plan of God (Acts 2 v23).
But it is the duty of men and women, families and nations, to openly and heartily acknowledge God as their king, both in word and practice. Left to ourselves, none of us actually does this, but God can and does change sinners through the Gospel of Jesus Christ.
God Subdues
The Gospel is the message of salvation, that Christ Jesus came into the world to save sinners. The message includes a command to all who hear to repent and trust in the Lord Jesus Christ to be saved from sin and guilt. Those who receive this message acknowledge Christ as saviour and as their Lord and king. But why do some receive the Gospel and bow the knee to the Christ of God, while others do not?
The answer is that God, who is sovereign over the will of man, fulfils his eternal plan to save a chosen multitude by making each one of them willing to believe at the time he has appointed. "Thy people shall be willing in the day of thy power" (Psalm 110 v3). "As many as were ordained to eternal life believed" (Acts 13 v48). It is this gracious and sovereign work of God in subduing sinners by renewing their proud hearts and wills to receive the Gospel that is prayed for in our text.
God Saves
If we pray 'thy kingdom come', we acknowledge that God saves not man. God
causes people to become real Christians. That's why the Apostle Paul prayed
for his countrymen: "Brethren, my heart's desire and prayer to God for Israel
is, that they might be saved" (Romans 10 v1).
Questions
Do you acknowledge Christ as your king? Do you desire to do so more
consistently by following the Bible more in daily life? Do you desire others
to submit to Christ through the Gospel? The answer to these questions tells
you whether you can pray, 'thy kingdom come'. It also tells you whether you
are a real Christian.