1 In this Psalm, we ought to seek in the whole body of it what we find in the first verse: “Mercy and judgment will I sing unto You, O Lord”. Let no man flatter himself that he will never be punished through God's mercy; for there is judgment also; and let no man who has been changed for the better dread the Lord's judgment, seeing that mercy goes before it. For when men judge, sometimes overcome by mercy, they act against justice; and mercy, but not justice, seems to be in them: while sometimes, when they wish to enforce a rigid judgment, they lose mercy.
But God neither loses the severity of judgment in the bounty of mercy, nor in judging with severity loses the bounty of mercy. Suppose we distinguish these two, mercy and judgment, by time; for possibly, they are not placed in this order without a meaning, so that he said not “judgment and mercy,” but “mercy and judgment:” so that if we distinguish them by succession in time, perhaps we find that the present is the season for mercy, the future for judgment. How is it that the season of mercy comes first?
Consider first how it is with God, that you also may imitate the Father, in so far as He shall permit you....“He makes His sun to rise on the evil and on the good, and sends rain on the just and on the unjust.” Behold mercy. When you see the just and the unjust behold the same sun, enjoy the same light, drink from the same founts, satisfied with the same rain, blessed with the same fruits of the earth, inhale this air in the same way, possess equally the world's goods; think not that God is unjust, who gives these things equally to the just and the unjust.
It is the season of mercy, not as yet of judgment. For unless God spared at first through mercy, He would not find those whom He could crown through judgment. There is therefore a season for mercy, when the long-suffering of God calls sinners to repentance.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)