Adam Clarke on Revelation 1:6

Kings and priests - See on 1Pet 2:5 (note), 1Pet 2:9 (note). But instead of βασιλεις και ἱερεις, kings and priests the most reputable MSS., versions, and fathers have βασιλειαν ἱερεις, a kingdom and priests; i.e. a kingdom of priests, or a royal priesthood. The regal and sacerdotal dignities are the two highest that can possibly exist among men; and these two are here mentioned to show the glorious prerogatives and state of the children of God.

To him be glory - That is, to Christ; for it is of him that the prophet speaks, and of none other.

For ever and ever - Εις τους αιωνας των αιωνων· To ages of ages; or rather, through all indefinite periods; through all time, and through eternity.

Amen - A word of affirmation and approbation; so it shall be, and so it ought to be.