Matthew incorporates this line, albeit with an asterisk as reminder to the reader that the earliest manuscripts did not contain these words.
Luke does not have the doxology at all.
So where did this liturgical addition come from?
1 Chronicles 29:10-13?
"Then David blessed the Lord in the presence of all the assembly; David said: ‘Blessed are you, O Lord, the God of our ancestor Israel, for ever and ever. Yours, O Lord, are the greatness, the power, the glory, the victory, and the majesty; for all that is in the heavens and on the earth is yours; yours is the kingdom, O Lord, and you are exalted as head above all. Riches and honor come from you, and you rule over all. In your hand are power and might; and it is in your hand to make great and to give strength to all. And now, our God, we give thanks to you and praise your glorious name."
This collection of early apostolic teachings prescribes disciples gathered and scattered thrice daily to recite this prayer, including the doxology.
There are probably as many suggestions as there are stars in the sky for the origin of the doxology that concludes the traditional rendition of Jesus' Prayer. Regardless of where the doxology comes from, the lyric is critical for liturgical, theological, and related missional purposes.
True. Jesus may not have said it when initially asked about how to pray. After all, he needed no reminders in regards to whom the kingdom, the power, and the glory belonged.
But I do.
The church does.
We all do.
Maybe that's why it was added later, lest we forget.
As I have worked through this late arrival to the Abba Prayer,  much of my reflections have been guided by Jan Milič Lochman. The Czech theologian delivered an address to the World Alliance of Reformed Churches in 1982, "Thine Is the Kingdom, and the Power and the Glory," and his words continue to echo relevance throughout a world saturated in unjust kingdoms, graceless powers, and quests for glory no matter what the cost.
So I find it best, as I muse about the doxology, to simply throw out a few snippets from the address and add my own commentary 30 years after these words were initially spoken in Ottawa, Ontario.
Thine Is the Kingdom...
We live in a world where our allegiance is constantly lobbied for by kingdoms, corporations, politicians, brands, and banks. "I pledge allegiance to the...market, media, flag, sale, image, or ideology." The temptation also looms large to think, even if just for a second, that we are the center of our own kingdom and the rest of the world exists to serve us and our needs.
"It makes all the difference to any culture or society if within it there are groups of people who keep their eyes open for the Kingdom of God in the midst of all the tribulations of their time, who seek first that Kingdom and its justice and do so, indeed, in the direction pointed by the promises of Christ: in the championship of the poor, in sacrificial service of the prisoners and the disabled, in comforting and encouraging the bruised and broken; and, above all, in proclaiming "the acceptable year of the Lord", the liberating future of God. For Thine Is the Kingdom!"
But, Thine is the kingdom...
And this kingdom's patrons are the poor, the oppressed, the weak, and the wounded. Citizens within this kingdom are championed within the opening statement of Jesus' hillside address in Matthew 5.
The poor. The hungry. The peacemakers. The meek. The sorrowful. The persecuted and oppressed.
When we pray, "Thine is the kingdom," we proclaim that our allegiance rests solely in this kingdom, the one Jesus announced as breaking in all around us.
Even more, we are reminded that while we may be covenant-partners  with God in faithful improvisations of this kingdom, it does not depend solely on us. It is not our kingdom. It is God's.
And even if we chose not to participate, the rocks would cry out and the kingdom would continue to come on earth as in heaven (Luke 19:40).
Thine Is the Power...
The kingdom of God wields an upside-down power. God's power is manifested in grace, sacrifice, love, justice, and a concern for the poor, oppressed, and marginalized of this world.
"A deliberate contrast is intended here, for when we ask what the Bible means by the power of God, it is at once clear that 'God has not joined the power club' (C.S. Song). If the power of God was revealed in the life of Jesus of Nazareth, if God's authoritative word took flesh and blood in Jesus, then it is equally clear that the direction in which this power moves is not towards graceless supremacy, compulsion and manipulation, not towards the establishment of dominance over others but towards their deliverance, their reclamation, their encouragement and consolation. 'For the Son of Man also came not to be served but to serve, and to give his life as a ransom for many.'"
God's power elevates the other.
God's power is recognized in weakness (2 Corinthians 12:9).
God's power forgives enemies.
God's power welcomes the stranger and immigrant.
God's power turns a first-century imperial execution device into a subversive movement towards the whole world's reconciliation, redemption, and salvation.
God's power is in the resurrection.
And it is God's power. Not mine. Not any of ours.
Not even the church's.
When we quest for and think we have attained and/or inherited power, we abuse it and exploit versus love our neighbors.
So thanks be to God that "thine is the kingdom and the power..." Let's leave it that way. Better said, when we do have "power," may we wield it in the same way God does (Philippians 2).
Thine Is the Glory...
Glory moves beyond abstraction. We must not think for a second that God sits upon a throne and awaits declarations of glory so to bask in human praise and adoration as though God had an inflated ego in constant need of being refueled.
"In the biblical view, these two passions—the passion for the glory of God and the passion for human welfare—go together. They are inseparably interwoven in the life and ministry of Jesus Christ. That life and ministry was wholly devoted to the glory of God and, at the same time, wholly given up to the sacrificial service of human beings in their diverse needs."
Instead, the glory of God is incarnated. God is glorified not only because God is the Creator of all things, but also because God is the One who sustains and reconciles all things (Colossians 1:15-20). And God does this by entering into the human condition, entering into the condition of the whole world, and working towards the world's total deliverance. God is glorified because God can and will make all things new.
This is the way of God's kingdom. This is the function of God's power. This is reason for God to be glorified.
The Lord's prayer is a beautiful declaration of dependence upon God as Father, Son, and Holy Spirit. Jesus' prayer is means to pledge faithful allegiance to the already and yet-to-come kingdom of God. And whenever we recite this prayer, maybe thrice daily, we are reminded of who we are, to whom we belong, and for what purposes we are called and sent. 
Lochman concluded his address to the World Alliance of Reformed Churches with a bold reminder:
Our life and witness, even our prayers, are to hinge on doxology. We love, serve, preach, teach, advocate, and liberate for no other reason than doxology. We love our neighbor as ourself because we first love God who first loved us.
"What is involved, therefore, when we pray "Thine is the Kingdom, the Power and the Glory" and seek to live accordingly is just this: justice, peace and joy in the Holy Spirit. In the glorification of God, none of these elements can be left out; not the championship of justice, not the commitment to the cause of peace. But it is only in the doxology of joy that our striving for justice and our commitment for peace achieve their goal, for the chief end of human life is to glorify God and to enjoy Him forever."
We are a doxological people. 
I am not sure where the doxology came from or how it was validated within the liturgy of Christian communities.
I am sure there are many who still question its source and authority.
I, on the other hand, am quite comfortable with this lyric.
Actually, I am grateful it's there.
Our Father who art in heaven, hallowed be thy name.Thy kingdom come, thy will be done, on earth as it is in heaven. Give us this day our daily bread; and forgive us our debts, as we forgive our debtors; and lead us not into temptation, but deliver us from evil. For thine is the kingdom and the power and the glory, forever. Amen.
 N.T. Wright has an excellent piece where he works through the Lord's Prayer. He often refers to the prayer as the Abba Prayer. See "The Lord's Prayer as a Paradigm on Christian Prayer."
 Karl Barth said it this way, “[The verdict of God] declares that God receives man, and that man in accordance with his election and institution as a covenant-partner- can confess himself a faithful servant of God, His recognized friend and well-loved child” (Church Dogmatics IV, 94).
 N.T. Wright writes in the above mentioned article, "The Lord’s Prayer is thus a marker, a reminder, to the church of who it is and why."
 I cannot claim this line. One of our Associate Pastors referred tot he church as a doxological people in morning prayer service this past week. Beautiful, Gary!
 See also one of my recent sermons: Forgiveness As Jubilee
 Jürgen Moltmann also has a great sermon delivered during opening worship at this same WARC gathering. So wish I was at this conference...but alas, I was not even born...