In the third chapter of the Book of Jonah, Jonah finally reaches Nineveh, and starts preaching repentance and reformation to the people there. And... they listen! Even the King of Assyria listens!

The inner meaning of the story is pretty close to the surface here, and there's important symbolism. Swedenborg summarizes the chapter's meaning in this one sentence:

"The nations, hearing from the Word of God about their sins, and that they would perish, were converted after repenting, and were heard by the Lord, and saved." (The Inner Meaning of the Prophets and Psalms 213)

Rev. George McCurdy, in his exegesis of this chapter, offers this summary:

The great city of Nineveh has many spiritual representations. The world saw Nineveh as a mighty nation, but in the eyes of the Lord, it represented the shallowness and weakness (death) of false doctrines, contrived beliefs confirmed by the proprium (self love), the fallacies of the senses, and the unenlightened understanding that spawns spiritual ignorance.

The Lord’s mercy and love for all prompted Him not only to send His message to Nineveh, but to ensure that it would be preserved for eternity. Jonah’s call to "arise and go" preach to Nineveh is a call for spiritual renewal. It is a call for repentance. The fasting, sackcloth, and ashes symbolize a call to be aware that "where there is no truth, there is no church."

Where the Word is closed, and not loved, there is a loss of conjunction with the Divine resources. The fasting of man and beast symbolizes a need for the spiritual and natural appetites to come to the Lord for that "bread which comes down from heaven." The Ninevites giving up their natural food and drink represents their turning away from "as-of-self" concepts and reminds all who read and hear the Word of the Lord to rely on heavenly manna.

"Why do you spend money for what is not bread, and your wages for what does not satisfy? Listen carefully to Me and eat what is good, and let your soul delight itself in abundance…" (Isaiah 55:2)

The king sitting in ashes and mourning represents a sorrowful attitude toward "the remains of the fire of self-love." Repentance involves self-examination, making oneself guilty for the sins and evils committed, asking the Lord for His help, and desiring to attain a new life.

As we mull over this concept, think about the Lord’s preaching on earth. As He went about His mission, He often used these words: "You have heard it said of old… but I say unto you…" Putting aside human traditions and taking on the Lord’s teachings is an ongoing effort. Jonah was told to preach the words of the Lord. He was not to make things up from his memory. He was to be a true and faithful prophet of the Lord’s ways.

Here's a link to an interesting (audio) sermon on this chapter, by Rev. Todd Beiswenger.

Rev. Martin Pennington suggests two explanatory passages from Swedenborg's theological writings:

"Forty days means a period of temptation combats or severe change." (Arcana Coelestia 730)

"A fast represents those who are in a state of unhappiness when good is no longer joined to the truths." (Arcana Coelestia 9182[10])

Other references of interest listed by Rev. McCurdy include:

"As ‘forty’ signified what is full or complete… forty signifies what is complete in respect to instruction and influx… for this reason it was said by Jonah to the Ninevites that ‘the city would be overthrown after forty days" (Jonah 3:4) (Arcana Coelestia 9437[2])

"By being clothed in sackcloth is signified mourning on account of the vastated truth in the church; for garments signify truths; and therefore, to be clothed in sackcloth, which is not a garment, signifies mourning that there is no truth; and where there is no truth, there is no church." Jonah 3:5-6 is cited. (Apocalypse Revealed 492)

"…the rite of putting sackcloth on the loins to testify…mourning may be seen from the historic and prophetic parts of the Word, as in… Jonah 3:5-8… for a sign representative of mourning over the evil on account of which Nineveh was to perish; thus over destroyed good." (Arcana Coelestia 4779[2])

"From the signification of garments it is also manifest why they rent their garments, when any one spoke against the Divine truth of the Word… and why, on account of transgressions against Divine truths, they put off their garments, and put on sackcloth." Jonah 3:5, 6, and 8 are cited. (Apocalypse Revealed 166)

"That ‘ashes’ denote falsity, may be confirmed from passages where another word for ‘ashes (cinis)’ is used, for these ashes have a like origin…" Jonah 3:6 is cited. (Arcana Coelestia 7520)

"…by ‘ashes’ in the opposite sense, namely, what is condemned that remains after the burning from the fire of self love. This is signified by ‘the ashes’ which they carried on the head, and in which they rolled themselves when bewailing their sins." Jonah 3:6 is cited. (Arcana Coelestia 9723)

"In the spiritual sense by kings those who are in truths are signified, by the great ones those who are in goods, by the rich those that are in the knowledge of good, by the mighty they that are in erudition, by servants they that are in such things from others, and thus from memory, and by freemen they that are in such things from themselves…" Jonah 3:7 is cited. (Apocalypse Revealed 337)

"The interior good and also the interior evil…are signified by ‘man,’ are those which are of the intention or end, for the intention or end is the inmost of man; but the exterior good and also the exterior evil which are signified by ‘beast,’ are those which are of the thought, and of the consequent action when nothing stands in the way…By ‘beast,’…in respect to the external or natural man, a man is nothing else than a beast, for he takes delight in the like cupidities and pleasures, as also in the like appetites and sense…Therefore… from a holy rite it was commanded by the king of Nineveh, that both man and beast were to fast, and were to be covered with sackcloth (Jonah 3:7, 8)." (Arcana Coelestia 7523)

"… in the spiritual world a man’s affections appear at a distance like beasts… and beasts, viewed in themselves, are nothing but forms of natural affections… By man and beasts together is signified man as to spiritual and natural affection, in the following passages… Jonah 3:7, 8…" (Apocalypse Revealed 567)

"Here ‘webs’ and ‘garments’ are predicated of things of the understanding, that is, of the thought; ‘iniquity’ and ‘violence,’ of things of the will, that is, of works. In Jonah 3:8… the ‘evil way’ is predicated of falsities, which are of the understanding; and ‘violence,’ which are of the will." (Arcana Coelestia 623[2])

"…the ‘wrath of anger’ is attributed to Jehovah, and consequently ‘repentance.’" (Arcana Coelestia 588[2])

"…the Word is such in the sense of the letter, it may be evident that it cannot be understood without doctrine. But let examples illustrate this. It is said that Jehovah repenteth (Jonah 3:9; 4:2), and it is also said that Jehovah repenteth not (Num. 23:19…): without doctrine these statements do not agree." (Teachings Regarding the Sacred Scripture 51)

"In these passages (Jonah 3:9,10) Jehovah is said to have ‘repented,’ when yet it cannot be that He repents, because He knows all things before He does them; from which it is evident that by ‘repenting’ is signified mercy." (Arcana Coelestia 10441[2-4])

Finally, here's a link to Rev. McCurdy's study guide for the Book of Jonah, which is available for free as a .pdf, for your use.

402. 'A city that was built' means all doctrinal or heretical teaching founded on that heresy. This is clear from the Word wherever the name of any city occurs. In the Word 'city' never means a city but something doctrinal or else something heretical. For angels are totally ignorant of what a city is or what the name of any city is. They never do nor can have any city in mind, for their ideas are of spiritual and celestial things, as shown already. Their perception is solely of what is meant spiritually by cities, and the names of them. For example, by the Holy City, which is also called the Holy Jerusalem, they understand nothing other than the Lord's kingdom in general, or as it exists with each individual who has the Lord's kingdom within him. And the city of Zion or Mount Zion they understand in a similar way, the latter being the celestial degree of faith, the former the spiritual.

[2] And the celestial and spiritual itself is also described by cities, palaces, houses, walls, the foundations of walls, ramparts, gates, bars, and by the temple at the centre, as in Ezekiel 48, and in Revelation 21:15-end. In Revelation 21:2, 10, it is called 'the Holy Jerusalem'; in Jeremiah 31:38 ['the city for Jehovah']; in David, Psalms 46:4, 'the city of God, the holy place of the dwellings of the Most High'; and in Ezekiel 48:35, it is called 'the city, Jehovah is there'. And in Isaiah,

The sons of the foreigner will build up your walls. They will bend down to the soles of your feet, all who disapprove of you, and they will call you the City of Jehovah, the Zion of the Holy One of Israel. Isaiah 60:10, 14.

In Zechariah,

Jerusalem [will be called] the city of truth, and Mount Zion the mountain of holiness. Zechariah 8:3

Here 'city of truth', which is Jerusalem, means the spiritual things of faith, and 'the holy mountain', which is Zion, the celestial things of faith. And whereas the celestial and spiritual things of faith were represented by a city, so all matters of doctrine were meant by the cities of Judah and Israel, each one, when mentioned by name, meaning some specific point of doctrine, though exactly which nobody can know except from the internal sense.

[3] As cities meant matters of doctrine, cities also meant heretical ideas, each one when mentioned by name meaning some specific heretical idea. But at this point solely the consideration that in general a city means doctrinal teaching or else heretical may be established from the following places:

[4] In Isaiah,

On that day there will be five cities in the land of Egypt which speak in the lip of Canaan and swear to Jehovah Zebaoth. One of these will be called the city Heres. Isaiah 19:18.

This refers to man's knowledge of spiritual and celestial things at the time of the Lord's Coming. In the same prophet,

Full of tumults, a tumultuous city, an exultant city. Isaiah 22:1, 2.

This refers to 'the valley of vision', which is delusion. In Jeremiah,

The cities of the south are shut up, with none opening them. Jeremiah 13:10.

This refers to people who are in 'the south', that is, who dwell in the light of truth, but blot it out. In the same prophet,

Jehovah thought to destroy the wall of the daughter of Zion. He causes rampart and wall to mourn; they have languished together. Her gates have sunk into the ground, He has destroyed and broken in pieces her bars. Lamentations 2:8-9.

Here anyone may see that nothing else is meant by 'wall, rampart, gates and bars' than matters of doctrine.

[5] Similarly in Isaiah,

This song will be sung in the land of Judah, Ours is a strong city, salvation will establish walls and a rampart. Open the gates that the righteous nation that keeps faith may enter in. Isaiah 26:1-2.

In the same prophet,

I will exalt You, I will confess Your name. You have made the city into a heap, the fortified city into a ruin; let not a palace of aliens be built of the city for ever. Therefore a strong people will honour You, the city of terrifying nations will fear You. Isaiah 25:1-3.

Nor does this refer to any actual city. In Balaam's prophecy,

Edom will be an inheritance, and out of Jacob one will have dominion, and he will accomplish the destruction of the remnant of the city. Numbers 24:18-19.

Here anyone may see that 'the city' does not mean an actual city. In Isaiah,

The city of hollowness has been broken down, every house has been shut up so that none may enter in. There is an outcry in the streets over the wine. Isaiah 24:10-11.

Here 'city of hollowness' stands for hollowness of doctrine. In this and other places 'streets' means the things that constitute a city, namely falsities or truths. In John,

When the seventh angel poured out his bowl the great city was split into three parts and the cities of the nations fell. Revelation 16:17, 19.

That 'a great city' means something heretical, as do 'the cities of the nations', may be clear to anyone. The explanation is also given in Revelation 17:18 that the great city means the woman whom John saw, 'the woman', as shown already, being a Church of that nature.