This doxology, or ascription of praise, is connected with the prayer by the word "for," to signify that all these things--the reign, power, and glory of God--will be manifested by granting these petitions. It is not because we are to be benefited, but that God's name and perfections may be manifested. His glory is, then, the first and principal thing which we are to seek when we approach him. We are to suffer our concerns to be sunk and lost sight of in the superior glory and honour of his name and dominion. We are to seek temporal and eternal life, chiefly because the honour of our Maker will be promoted, and his name be more illustriously displayed to his creatures. He is to be "first, last, supremest, best," in our view; and all selfish and worldly views are to be absorbed in that one great desire of the soul that God may be "all in all." Approaching him with these feelings, our prayers will be answered, our devotions will rise like incense, and the lifting up our hands will be like the evening sacrifice.
Amen. This is a word of Hebrew origin, from a verb signifying to be firm, secure, to be true and faithful. It is a word expressing consent or strong approbation, a word of strong asseveration. It means verily, certainly, so be it. It is probable that this word was used by the people in the synagogue to signify their assent to the prayer that was uttered by the minister. And to some extent, it was probably so used in the Christian church. See 1Cor 14:16. It may be proper to remark, that this doxology, "for thine is the kingdom," etc., is wanting in many manuscripts, and that its authenticity is doubtful.
(b) "from evil" Jn 17:15 (c) "For thine" Rev 5:12,13
Source: Barnes' New Testament Notes