While rushing to enter the Wicket-gate on Evangelist's urging, Christian (initially joined by Pliable) accidentally plunges into a miry bog called the Slough of Despond. The visual imagery of a squelchy, muddy struggle conveys the desperation that a Christian, newly persuaded of their sinful state, often faces:
Now I saw in my Dream, that [...] they drew near to a very miry Slough, that was in the midst of the plain and they, being heedless, did both fall suddenly into the bog. The name of the slough was Dispond. Here therefore they wallowed for a time, being grievously bedaubed with the dirt; and Christian, because of the Burden that was on his back, began to sink in the mire.
The narrator observes that Christian and Pliable fall into the Slough because they are "heedless," presumably not watching carefully enough where they're going. But once they're in the bog, the surroundings fit its name, Despond, which connotes deep discouragement. The men "wallow[]," are "grievously bedaubed with dirt," and Christian even sinks. Being completely covered with mud, to the point of hardly being recognizable and being unable to keep one's bearings, is also an allegorical depiction not just of the polluting nature of sin but of what happens when a person is so burdened by their consciousness of sin that they feel trapped by it, totally helpless to pull themselves out of the dirt, much less make their way forward.
Later, after a passerby named Help pulls Christian out of the Slough, he explains why the bog has formed here:
This miry Slough is such a place as cannot be mended; it is the descent whither the scum and filth that attends conviction for sin doth continually run, and therefore it is called the Slough of Dispond; for still as the sinner is awakened about his lost condition, there ariseth in his soul many fears and doubts, and discouraging apprehensions, which all of them get together, and settle in this place: And this is the reason of the badness of this ground.
Help's explanation contains further dirty imagery, namely "scum and filth" pouring into a great ditch and accumulating into a stagnant, smelly swamp. He also deepens the allegory by explaining how, when a person feels "convicted" or convinced of their sin and "lost condition" relative to God and heaven, fears, doubts, and discouragements settle in their conscience much as mud collects and builds up in a miry bog. Help suggests to Christian that, as long as there are sinners, the Slough of Despond will always exist and pose a threat to those sinners. Some, like Pliable, will quickly grow disheartened and flee back to Destruction; others, like Christian, must rely on fellow pilgrims' help to extricate them from the muck and help them get on their way. So forewarned, readers might be able to dodge the "slough" of doubt and discouragement in their own lives, or at least recognize it when they stumble into it and not despair.
Early in his pilgrimage, Christian visits the Interpreter's House, where the Interpreter explains various symbols pertaining to the spiritual life. In one of the Interpreter's lessons, Bunyan uses the imagery of a dusty parlor to show the state of a heart corrupted by sin:
This parlour is the heart of a man that was never sanctified by the sweet Grace of the Gospel: the dust is his Original Sin and inward Corruptions, that have defiled the whole man. He that began to sweep at first, is the Law; but she that brought water, and did sprinkle it, is the Gospel. Now, whereas thou sawest that so soon as the first began to sweep, the dust did so fly about that the Room by him could not be cleansed, but that thou wast almost choaked therewith; this is to shew thee, that the Law, instead of cleansing the heart (by its working) from sin, doth revive, put strength into, and increase it in the soul, even as it doth discover and forbid it, for it doth not give power to subdue.
The dust in the parlor symbolizes "original sin" (the sinful nature passed down to all of humanity from Adam and Eve), as well as all the "corruptions" that proceed from original sin, inclining a person toward evil in everything they do. Note that when a man comes into the dusty parlor and begins to sweep, his effort backfires: the dust gets stirred up to such a degree that Christian chokes on it. The Interpreter tells Christian that the dust represents God's Law's action in a sinful heart. That is, the harder a person tries to obey God's Law (as written in the Old Testament), the more they sin, not less. The imagery of whirling, choking dust conveys a sinful heart's sheer helplessness to rid itself of corruption by its own efforts. As the Interpreter puts it, the Law "doth not give power to subdue," or overcome sin. It can only reveal just how pervasive sin is, as even the most well-intended person inevitably stumbles into further transgression.
Later, a girl sprinkles water on the dusty parlor, which symbolizes the gospel's grace. From Bunyan's Protestant perspective, this sprinkled water specifically symbolizes Jesus Christ's blood, which is the only thing that can cleanse corruption from a sinner's heart and redeem them.
Bunyan uses the imagery of the man's frantic, ineffectual sweeping to impress on readers a sinner's inability to cleanse their own heart of corruption. Only a remedy applied from the outside—namely, the sprinkling of Christ's sinless blood—can effectively clean up the "dust" of sin.
Some of the book's most moving imagery appears at the climax of Part 1, when Christian and Hopeful finally enter the Celestial City. Bunyan uses visual and auditory imagery to entice readers with the promised joys of heaven:
[...] they were transfigured, and they had Raiment put on that shone like Gold. There was also that met them with Harps and Crowns, and gave them to them, the Harps to praise withal, and the Crowns in token of honour. Then I heard in my Dream that all the Bells in the City rang again for joy, and that it was said unto them, Enter ye into the joy of your Lord. [...]
[...] I looked in after them, and behold, the City shone like the Sun: the Streets also were paved with Gold, and in them walked many men, with Crowns on their heads, Palms in their hands, and golden Harps to sing praises withal.
The narrator slows down and describes what he sees and hears in his dream in detail. To an audience familiar with biblical language, "transfigured" evokes an incredibly bright, shining appearance. Christian and Hopeful also receive clothing that "shone like gold"; the City itself "shone like the Sun," the streets are golden, and so are the people's harps (presumably their crowns, too!). Altogether, this visual imagery makes the pilgrims' new home sound almost too stunningly bright to look at directly. Joyfully pealing bells, a voice joyfully quoting the Bible, and crowds of people praising God with harp music add to the sense that the Celestial City rings with a kind of heavenly cacophony.
Of course, imagery like harps, crowns, palm branches, and streets of gold also have strong biblical associations. After the plodding, often painful hardships of pilgrimage, these familiar, gleaming, and joyful images invite readers to celebrate with Christian and Hopeful and are meant to make them, too, feel as though they've finally made it—indeed, to make them anticipate their own arrival in heaven one day.
