Jonah 3:5

"So the people of Nineveh believed God, and proclaimed a fast, and put on sackcloth, from the greatest of them even to the least of them." LXX: similar. Nineveh believed but Israel did not believe; the foreskin believed, but circumcision remained without faith. First of all the men of Nineveh believed who had arrived at the age of Christ [Eph. 4:13]: they announced a fast and dressed in sackcloth, from the greatest to the smallest of them. This regime and clothing is very worthy of penitence, so that those who had offended God through their indulgence or lust appeased him by condemning all that they had previously offended with. Sackcloth and fasting are the weapons of penitence, the rescue of sinners. First of all fasting, then sackcloth; first of all what is not seen, then what is visible; the one is always shown to God, the other sometimes to man. And if it were necessary to remove one from the two then I would rather keep fasting without sackcloth than have sackcloth without fasting. Elder men give the example which pertains to youths: for no one is without sin; and if his life only lasted one day, the years of his life would still be counted [Job 14:5. LX]. For if the stars are not pure before God, they are still more so than a worm or putrefaction, and those who are held by the sin of Adam, the great offender. Note here too the order, which is well written: God commands the prophet, the prophet proclaims to the city. First of all the men believe, announce fasting, and then everyone puts on sackcloth. The men do not announce the putting on of sackcloth, but only the fasting. All the same, with reason, those to whom penitence has been proscribed wear sackcloth and fast so that empty stomach and mourning clothes give the Lord more of an opportunity to remit.

But the Chaldaeans and the Medes and Persians, having a somewhat wider knowledge, were instructed by the building of the Tower, and the deluge, and by what happened in the case of Hezekiah and Jonah, and by the Captivity, and by Daniel and the Three …

Let us turn to every age that has passed, and learn that, from generation to generation, the Lord has granted a place of repentance to all such as would be converted unto Him... Jonah proclaimed destruction to the Ninevites; but they, repenting of their sins, propitiated God by prayer, and obtained salvation, although they were aliens [to the covenant] of God.

Do you see how vexed God is when fasting is treated despitefully? Learn how delighted he is when fasting is honored. When Eve was maltreated, he inflicted death as a penalty upon the insolent individual. He revoked death when she was honored once again. Desiring to show you the power of this thing of importance, he gave her authority over the sentence, after the arrest, to snatch the prisoners from the middle of the journey and change their course toward life. And he did this not only for two or three or twenty people but also for a whole population, in the case of the great and marvelous city of the Ninevites, which had knelt and bowed its head over this pit of perdition and was expecting to suffer the blow from above. Like a heavenly power overseeing Nineveh's charge, fasting snatched the city from these gates of death and returned Nineveh to life.

Let us sow in tears, so that we may reap in joy. Let us show ourselves people of Nineveh, not of Sodom. Let us amend our wickedness, lest we be consumed with it. Let us listen to the preaching of Jonah, lest we be overwhelmed by fire and brimstone. And if we have departed from Sodom, let us escape to the mountain. Let us flee to Zoar. Let us enter it as the sun rises. Let us not stay in all the plain. Let us not look around us, lest we be frozen into a pillar of salt, a really immoral pillar, to accuse the soul that returns to wickedness.