The conclusion: For thine is the kingdom, and the power and the glory, for ever. Amen. Some refer this to David's doxology, 1Ch 29:11. Thine, O Lord, is the greatness. It is,
1. A form of plea to enforce the foregoing petitions. It is our duty to plead with God in prayer, to fill our mouth with arguments (Job 23:4) not to move God, but to affect ourselves; to encourage the faith, to excite our fervency, and to evidence both. Now the best pleas in prayer are those that are taken from God himself, and from that which he has made known of himself. We must wrestle with God in his own strength, both as to the nature of our pleas and the urging of them. The plea here has special reference to the first three petitions; "Father in heaven, thy kingdom come, for thine is the kingdom; thy will be done, for thine is the power; hallowed be thy name, for thine is the glory." And as to our own particular errands, these are encouraging: "Thine is the kingdom; thou hast the government of the world, and the protection of the saints, thy willing subjects in it;" God gives and saves like a king. "Thine is the power, to maintain and support that kingdom, and to make good all thine engagements to thy people." Thine is the glory, as the end of all that which is given to, and done for, the saints, in answer to their prayers; for their praise waiteth for him. This is matter of comfort and holy confidence in prayer.
Source: Matthew Henry's Commentary on the Whole Bible Volume V (Matthew to John)
Thine is the kingdom. That is, thine is the reign or dominion. Thou hast control over all these things, and canst so order them as to answer these petitions.
Source: Barnes' New Testament Notes