6 When therefore He has sent the rod of His power out of Sion: what shall happen? “Be Thou ruler, even in the midst among Your enemies.” First, “Be Thou ruler in the midst of Your enemies:” in the midst of the raging heathen. For shall He rule “in the midst of His enemies” at a later season, when the Saints have received their reward, and the ungodly their condemnation? And what wonder if He shall then rule, when the righteous reign with Him for ever, and the ungodly burn with eternal punishments? What wonder, if He shall then? Now “in the midst of Your enemies,” now in this transition of ages, in this propagation and succession of human mortality, now while the torrent of time is gliding by, unto this is the rod of Your power sent out of Sion, “that You may be Ruler in the midst of Your enemies.” Rule Thou, rule among Pagans, Jews, heretics, false brethren. Rule Thou, rule, O Son of David, Lord of David, rule in the midst of Pagans, Jews, heretics, false brethren. “Be Thou Ruler in the midst of Your enemies.” We understand not this verse aright, if we do not see that it is already going on....
7. “With You the beginning on the day of Your power”. What is this day of His power, when is there beginning with Him, or what beginning, or in what sense is there beginning with Him, since He is the Beginning?...
Source: The Enarrations, or Expositions, on the Psalms (New Advent)