4 But seeing that he has changed the order of the words (though he had first said, “O God, Your judgment to the King give Thou, and Your justice to the King's Son,” putting judgment first, then justice), and has put justice first, then judgment, saying, “To judge Your people in justice, and Your poor in judgment:” he does more clearly show that he has called judgment justice, proving that there is no difference made by the order in which the word is placed, because it signifies the same thing. For it is usual to say “wrong judgment” of that which is unjust: but justice iniquitous or unjust we are not wont to speak of. For if wrong and unjust it be; no longer must it be called justice. Again, by putting down judgment and repeating it under the name of justice, or by putting down justice and repeating it under the name of judgment, he clearly shows that he specially names that judgment which is wont to be put instead of justice, that is, that which cannot be understood of giving an evil judgment. For in the place where He says, “Judge not according to persons, but right judgment judge ye;” He shows that there may be a wrong judgment, when He says, “right judgment judge ye:” lastly, the one He does forbid, the other He does enjoin. But when without any addition He speaks of judgment, He would at once have just judgment to be understood: as is that which He says, “You forsake the weightier matters of the Law, mercy and judgment.” That also which Jeremiah says is, “making his riches not with judgment.” He says not, making his riches by wrong or unjust judgment, or not with judgment right or just, but not with judgment: calling not anything judgment but what is right and just.
5. “Let the mountains bear peace to the people, and the hills justice”. The mountains are the greater, the hills the less. These are without doubt those which another Psalm has, “little with great.” For those mountains did exult like rams, and those hills like lambs of the sheep, at the departure of Israel out of Egypt, that is, at the deliverance of the people of God from this world's servitude. Those then that are eminent in the Church for passing sanctity, are the mountains, who are meet to teach other men also, by so speaking as that they may be faithfully taught, by so living as that they may imitate them to their profit: but the hills are they that follow the excellence of the former by their own obedience. Why then “the mountains peace: and the hills justice”? Would there perchance have been no difference, even if it had been said thus, Let the mountains bear justice to the people and the hills peace? For to both justice, and to both peace is necessary: and it may be that under another name justice herself may have been called peace. For this is true peace, not such as unjust men make among them. Or rather with a distinction not to be overlooked must that be understood which he says, “the mountains peace, and the hills justice”? For men excelling in the Church ought to counsel for peace with watchful care; lest for the sake of their own distinctions by acting proudly they make schisms and dissever the bond of union. But let the hills so follow them by imitation and obedience, that they prefer Christ to them: lest being led astray by the empty authority of evil mountains (for they seem to excel), they tear themselves away from the Unity of Christ....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)