-from a commentary on the book of Joel by Saint Jerome, priest, Doctor of the Church (PL 25, 967-968) as found in the Liturgy of the Hours, Office of Readings, Second Reading, 21st Week in Ordinary Time.
“”Return to me with all your heart [Joel 2:12] and show a spirit of repentance with fasting, weeping and mourning [Joel 2:12]; so that while you fast now, later you may be satisfied, while you weep now, later you may laugh, while you mourn now, you may some day enjoy consolation [cf Luke 6:21; Matthew 5:4]. It is customary for those in sorrow or adversity to tear their garments. The gospel records that the high priest did this to exaggerate the charge against our Lord and Savior; and we read that Paul and Barnabas did so when they heard words of blasphemy. I bid you not to tear your garments but rather to rend your hearts [Joel 2:13] which are laden with sin. Like wine skins, unless they have been cut open, they will burst of their own accord. After you have done this, return to the Lord your God, from whom you had been alienated by your sins. Do not despair of his mercy, no matter how great your sins, for great mercy will take away great sins [cf Luke 7:41-47].
For the Lord is gracious and merciful [Joel 2:13] and prefers the conversion of a sinner rather than his death. Patient and generous in his mercy, he does not give in to human impatience but is willing to wait a long time for our repentance. So extraordinary is the Lord’s mercy in the face of evil, that if we do penance for our sins, he regrets his own threat and does not carry out against us the sanctions he had threatened. So by the changing of our attitude, he himself is changed. But in this passage we should interpret “evil” to mean, not the opposite of virtue, but affliction, as we read in another place: Sufficient for the day are its own evils [cf Matthew 6:34]. And, again: If there is evil in the city, God did not create it.
In like manner, given all that we have said above – that God is kind and merciful, patient, generous with his forgiveness, and extraordinary in his mercy toward evil – lest the magnitude of his clemency make us lax and negligent, he adds this word through his prophet: Who knows whether he will not turn and repent and leave behind him a blessing? [Joel 2:14]. In other words, he says: “I exhort you to repentance, because it is my duty, and I know that God is inexhaustibly merciful, as David says: Have mercy on me, God, according to your great mercy, and in the depths of your compassion, blot out all my iniquities [cf Psalm 51:1]. But since we cannot know the depth of the riches and of the wisdom and knowledge of God, I will temper my statement, expressing a wish rather than taking anything for granted, and I will say: Who knows whether he will not turn and repent? [cf Joel 2:14]. Since he says, Who, it must be understood that it is impossible or difficult to know for sure.
To these words the prophet adds: Offerings and libations for the Lord our God [cf Joel 2:14]. What he is saying to us in other words is that, God having blessed us and forgiven us our sins, we will then be able to offer sacrifice to God.”