3 In fear of which condemnation the Church prays in this Psalm, and says, “Reprove me not, O Lord, in Your anger”. The Apostle too mentions the anger of the judgment; “You treasure up unto yourself,” he says, “anger against the day of the anger of the just judgment of God.” In which he would not be reproved, whosoever longs to be healed in this life. “Nor in Your rage chasten me.” “Chasten,” seems rather too mild a word; for it avails toward amendment. For for him who is reproved, that is, accused, it is to be feared lest his end be condemnation.
But since “rage” seems to be more than “anger,” it may be a difficulty, why that which is milder, namely, chastening, is joined to that which is more severe, namely, rage. But I suppose that one and the same thing is signified by the two words. For in the Greek θυμὸς, which is in the first verse, means the same as ὀ ργὴ, which is in the second verse. But when the Latins themselves too wished to use two distinct words, they looked out for what was akin to “anger,” and “rage” was used.
Hence copies vary. For in some “anger” is found first, and then “rage:” in others, for “rage,” “indignation” or “choler” is used. But whatever the reading, it is an emotion of the soul urging to the infliction of punishment. Yet this emotion must not be attributed to God, as if to a soul, of whom it is said, “but Thou, O Lord of power, judgest with tranquillity.” Now that which is tranquil, is not disturbed. Disturbance then does not attach to God as judge: but what is done by His ministers, in that it is done by His laws, is called His anger.
In which anger, the soul, which now prays, would not only not be reproved, but not even chastened, that is, amended or instructed. For in the Greek it is, παιδεύσῃς, that is, instruct. Now in the day of judgment all are “reproved” that hold not the foundation, which is Christ. But they are amended, that is, purged, who “upon this foundation build wood, hay, stubble. For they shall suffer loss, but shall be saved, as by fire.” What then does he pray, who would not be either reproved or amended in the anger of the Lord? What else but that he may be healed? For where sound health is, neither death is to be dreaded, nor the physician's hand with caustics or the knife.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)