15 “Your throne, O God, is for ever and ever”. Because God has “'blessed You' for ever,” on account of the “grace poured over Your lips.” Now the throne of the Jewish Kingdom was a temporal one; belonging to those who were under the Law, not to those who were under “grace:” He came to “redeem those who were under the Law,” and to place them under “Grace.” His “Throne is for ever and ever.” Why? For that first throne of the Kingdom was but a temporal one: whence then have we a “throne for ever and ever”?
Because it is God's throne. O divine Attribute of Eternity! for God could not have a temporal throne. “Your throne, O God, is for ever and ever— a sceptre of direction is the sceptre of Your Kingdom.” “The sceptre of direction” is that which directs mankind: they were before crooked, distorted; they sought to reign for themselves: they loved themselves, loved their own evil deeds: they submitted not their own will to God; but would fain have bent God's will to conformity with their own lusts.
For the sinner and the unrighteous man is generally angry with God, because it rains not! and yet would have God not be angry with himself, because he is profligate. And it is pretty much for this very reason that men daily sit, to dispute against God: “This is what He ought to have done: this He has not well done.” Thou forsooth see what you do. He knows not what He does! It is you that are crooked! His ways are right. When will you make the crooked coincide with the straight?
It cannot be made to coincide with it. Just as if you were to place a crooked stick on a level pavement; it does not join on to it; it does not cohere; it does not fit into the pavement. The pavement is even in every part: but that is crooked; it does not fit into that which is level. The will of God then is “equal,” your own is “crooked:” it is because you can not be conformed unto it, that it seems “crooked” unto you: rule you yourself by it; seek not to bend it to your own will: for you can not accomplish it; that is at all times “straight”!
Would you abide in Him? “Correct you yourself;” so will the sceptre of Him who rules you, be unto you “a rule of direction.” Thence is He also called King, from “ruling.” For that is no “ruler” that does not correct. Hereunto is our King a King of “right ones.” Just as He is a Priest (Sacerdos) by sanctifying us, so is He our King, our Ruler, by “ruling” us....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)