2. It is a form of praise and thanksgiving. The best pleading with God is praising of him; it is the way to obtain further mercy, as it qualifies us to receive it. In all our addresses to God, it is fit that praise should have a considerable share, for praise becometh the saints; they are to be our God for a name and for a praise. It is just and equal; we praise God, and give him glory, not because he needs it--he is praised by a world of angels, but because he deserves it; and it is our duty to give him glory, in compliance with his design in revealing himself to us. Praise is the work and happiness of heaven; and all that would go to heaven hereafter, must begin their heaven now. Observe, how full this doxology is, The kingdom, and the power, and the glory, it is all thine. Note, It becomes us to be copious in praising God. A true saint never thinks he can speak honourably enough of God: here there should be a gracious fluency, and this for ever. Ascribing glory to God for ever, intimates an acknowledgement, that it is eternally due, and an earnest desire to be eternally doing it, with angels and saints above, Ps 71:14.
Source: Matthew Henry's Commentary on the Whole Bible Volume V (Matthew to John)
Thine is the power. Thou hast power to accomplish what we ask. We are weak, and cannot do it; but Thou art almighty, and all things are possible with thee.
Thine is the glory. That is, thine is the honour or praise. Not our honour; but thy glory, thy goodness, will be displayed in providing for our wants; thy power, in defending us; thy praise, in causing thy kingdom to spread through the earth.
Source: Barnes' New Testament Notes