5 And what follows? “The Lord shall send the rod of Your power out of Sion”. It appears, brethren, it most clearly appears, that the Prophet is not speaking of that kingdom of Christ, in which He reigns for ever with His Father, Ruler of the things which are made through Him: for when does not God the Word reign, who is in the beginning with God? For it is said, “Now unto the King eternal, immortal, invisible, the only wise God, be honour and glory for ever and ever.” To what eternal King? To one invisible, incorruptible. For in this, that Christ is with the Father, invisible and incorruptible, because He is His Word, and His Power, and His Wisdom, and God with God, through whom all things were made; He is “King eternal;” but, nevertheless, that reign of temporal government, by which, through the mediation of His flesh, He called us into eternity, begins with Christians; but of His reign there shall be no end. His enemies therefore are made His footstool, while He is sitting on the right hand of His Father, as it is written; this is now going on, this will go on unto the end....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)