Soon after embarking on his pilgrimage, heading to the Wicket-gate at Evangelist's urging, Christian meets a man named Mr. Worldly Wiseman, who tells Christian to get his Burden removed by easier means—namely, by visiting a fellow named Legality. Worldly Wiseman's advice proves disastrous, however—en route, Christian becomes convinced that an overhanging mountain is going to fall and crush him—and an allusion to the biblical book of Galatians explains why. As Evangelist puts it,
He to whom thou was sent for ease, being by name Legality, is the Son of the Bond-woman which now is, and is in bondage with her children; and is in a mystery this Mount Sinai, which thou hast feared will fall on thy head.
Evangelist's explanation requires some unpacking. In Galatians 4, the Apostle Paul gives an allegory that hearkens back to the book of Genesis. In the allegory, the patriarch Abraham's concubine, the "bond-woman" Hagar, is associated with Mount Sinai, or the Law; and Abraham's wife, Sarah, is associated with the heavenly Jerusalem, from which salvation comes. Evangelist explains that Legality, whom Worldly Wiseman claimed could help Christian, is the bond-woman's son and therefore in bondage like her. That means Legality can't possibly free Christian from his burden—he's bound to the law (hence his name!), so how could he remove Christian's own burden of sin? In turn, the looming mountain that scared Christian represents Mount Sinai, where God first gave the Law to Moses. In short, Worldly Wiseman tricked Christian into thinking Legality was the answer to his problems, but Legality would only have compounded them. In a way, then, the dread Christian felt when he saw Mount Sinai warned him off from a more crushing fate.
The Galatians allegory is intricate and no doubt challenging for most readers to follow. However, Bunyan uses the allusion because he knows that most of his contemporary audience would have been familiar with the biblical passages involved and would have understood their theological implications. It's also an interesting example of an allegory being cited within an allegory—another instance where Bunyan uses well-known biblical allegories to subtly reinforce what he's doing more broadly in Pilgrim's Progress.
After lodging in the Palace Beautiful for a night, Christian is preparing to set off toward the Celestial City again. To encourage him and strengthen him on his way, the Palace's residents read him accounts of past pilgrims, which allude heavily to the biblical Book of Hebrews:
Then they read to him some of the worthy Acts that some of [Christ's] servants had done: as, how they had subdued Kingdoms, wrought Righteousness, obtained Promises, stopped the mouths of Lions, quenched the violence of Fire, escaped the edge of the Sword; out of weakness were made strong, waxed valiant in fight, and turned to flight the Armies of the Aliens.
From "subdued Kingdoms" to "Armies of the Aliens," this passage quotes directly from Hebrews 11:33-34. The broader context of Hebrews chapter 11 is the "great cloud of witnesses," a lengthy, dramatically building catalogue of allusions to Old Testament figures who persevered in their faith, enduring sufferings while they waited for the fulfillment of God's promises to them (i.e., the coming of Jesus Christ).
Most of Bunyan's original audience would have recognized this allusion instantly. They would have understood that Christian's companions read to him from the Bible in order to strengthen him for the obstacles he, too, will face before reaching Heaven. But it isn't just Christian's pilgrimage that builds off the example of his biblical forebears. With this allusion, Bunyan invites his audience, too, to regard themselves as part of this procession of biblical heroes who "stop[] the mouths of Lions" and "wax valiant in fight." In other words, the figures in Hebrews aren't just exemplars to read about; they are spiritual ancestors, and Bunyan's readers are meant to see themselves as members of the very same heavenward procession, carrying on the same legacy of suffering but persevering faith.
Just before entering the Valley of the Shadow of Death, Christian meets two men who are hurrying in the opposite direction—children of those who gave "an evil report of the good land." Men giving an "evil report" is an allusion to the Old Testament's book of Numbers. When Christian questions the men, they explain that they turned back from the Valley when they glimpsed its dangers:
CHR. But what have you seen? said Christian.
MEN. Seen! Why, the Valley itself, which is as dark as pitch; we also saw there the Hobgoblins, Satyrs, and Dragons of the Pit [...] Death also doth always spread his wings over it. In a word, it is every whit dreadful[.]
In the book of Numbers, Moses sends spies into the promised land of Canaan (which the people of Israel are about to enter) in order to report on the condition of the land and its people. Though the land is good and fertile and flows "with milk and honey," a handful of the returning spies give an "evil report," claiming that Canaan is filled with giants and that the men of Israel are like "grasshoppers" next to the Canaanites. The people of Israel are terrified at this news and balk at entering the promised land.
What's interesting about this allusion is that, in Christian's case, the land he's about to enter is much as the fleeing men say that it is—he goes on to find that it contains deep pits, impenetrable darkness, hellish noises, and demons whispering blasphemies—yet, unlike the Israelites on the border of the promised land, Christian doesn't hesitate to enter the Valley of the Shadow of Death. This suggests that, no matter how frightening the Valley might be for a little while, it's of no account when the Celestial City is so near. It's not that the men's "evil report" is without merit, but that the Valley is still a good place for Christian to be, if indeed it's the way to heaven.
Bunyan's use of the allusion to Numbers suggests that pilgrims on their way to heaven too readily turn back when they see dangers and difficulties ahead; they fail to understand that these dangers are ultimately no obstacle when a pilgrim has enough faith to face them head-on. Pilgrims shouldn't let rumors of hardship, no matter how well-founded, distract them from their goal.
During the final stage of their journey to the Celestial City, when Christian and Hopeful begin to cross over the River of Death, Christian, sinking, alludes to a Psalm:
They then addressed themselves to the Water; and entring, Christian began to sink, and crying out to his good friend Hopeful, he said, I sink in deep Waters; the Billows go over my head, all his Waves go over me[.]
Christian's cry is an allusion to Psalm 42:7—"Deep calleth unto deep at the noise of thy waterspouts: all thy waves and thy billows are gone over me." The verse, with its imagery of thunderous, raging waters closing over one's head, describes the experience of a person who believes in God yet feels that God has abandoned them to destruction.
The passage as a whole is also an allegory for the experience of death. In the novel, dying is pictured as the process of crossing the River of Death to the Celestial City (itself an allusion to the way God's people in the Old Testament crossed the River Jordan to enter the Promised Land). Yet the ease or difficulty of this experience varies from pilgrim to pilgrim. Here, Christian finds the crossing so terrifying that he doubts he will arrive safely at the other side. It takes his companion Hopeful's reminders that God is just testing his faith, not abandoning him, to eventually help Christian overcome his fears.
Yet by putting these words in Christian's mouth, Bunyan also reminds readers that the experience of feeling rejected by God isn't foreign to faithful pilgrims—it's right there in the Bible, after all. That means readers, as pilgrims themselves, have a pattern to follow even in the midst of their fears—remembering that God has proven to be a faithful helper in the past and that even when one's senses tell them they're sinking under the "Billows" and "Waves," they won't be lost.
During their stay at the Porter's House, Prudence asks Christiana's sons questions about their Christian beliefs, alluding to the common Protestant practice of catechizing children:
PRUDENCE. And she said, Come James, canst thou tell who made thee?
JAMES. God the Father, God the Son, and God the Holy Ghost.
PRUD. Good Boy. And canst thou tell me who saves thee?
JAMES. God the Father, God the Son, and God the Holy Ghost.
PRUD. Good Boy still. But how doth God the Father save thee?
JAMES. By his Grace.
PRUD. How doth God the Son save thee?
JAMES. By his Righteousness, Death, and Blood, and Life.
PRUD. And how doth God the Holy Ghost save thee?
JAMES. By his Illumination, by his Renovation, and by his Preservation.
Then said Prudence to Christiana, You are to be commended for thus bringing up your Children.
Prudence's questions and James's answers are an example of the catechism format, in which a parent, minister, or other authority figure would quiz a child on key religious teachings, and the child would recite memorized responses. Though catechisms varied by denomination, many followed a predictable shape, dealing with the three Persons of the Trinity (God the Father, God the Son, and God the Holy Ghost) and the benefits of salvation given by each. (Here, Bunyan actually quotes from a catechism he authored himself, not from a more famous one like the Westminster Shorter Catechism.) Even though it's doubtful that a young boy like James would understand the full meaning behind words like "illumination," "renovation," and "preservation," the practice of catechizing took a longer view—to provide a foundation for religious beliefs that a child would understand more deeply as they grew up. By alluding specifically to Christiana's role in "thus bringing up [her] children," Bunyan elevates women's, especially mothers', key role in the religious formation of young children within English-speaking Protestant cultures.
Earlier in the novel, Christiana's son Matthew had eaten fruit that belonged to the devil. Later, when the group is staying at the Porter's House, Matthew gets a terrible stomachache from the fruit, so, in Bunyan's metaphor for salvation from sin, an old doctor named Mr. Skill makes Matthew a:
[purge] to the purpose, ’twas made Ex Carne & Sanguine Christi. (You know Physicians give strange Medicines to their Patients.) And it was made up into Pills [...]. Now he was to take them three at a time fasting, in half a quarter of a pint of the Tears of Repentance.
There's a lot going on in this metaphor. First, ex Carne & Sanguine Christi is Latin for "from the body and blood of Christ." On a surface level, then, the purgative medicine could simply refer to Jesus Christ as the atoning sacrifice for Matthew's sin. On a more sophisticated level, it might be an allusion to the Christian sacrament of communion or the Eucharist, in which Christians believe that they partake of Christ's body and blood. Either or both references could be in play here, though given the narrator's aside about "strange Medicines," it's safe to assume that Bunyan is sticking to the more literal level. His greater point, after all, is that it's because of Christ's literal death that a sinner like Matthew (or any sinner) can be forgiven and "purged" of sin's effects.
In terms of Bunyan's Protestant theology, it's important to note that these special pills are only effective if taken with "Tears of Repentance," meaning that the pills won't work alone, but that the "patient" must be genuinely sorry for their sins in order for the healing effects of Christ's sacrifice to apply to them. The requirement to take the pills "fasting," or on an empty stomach, might be a further reference to the need for deep sorrow over sin, or it might also imply that the one who takes the pills shouldn't rely on any other remedy to heal them. The prescribed dose of three pills may be an allusion to the Holy Trinity, as is so often the case with symbolic "threes" in Christian literature.
Overall, the pills serve as a metaphor for the way a Christian is saved from sin—that is, by receiving Christ's sacrifice as one's only cure, something that can only be done with a repentant attitude. Though the metaphor appears in the context of a specific wrongdoing (Matthew eating the devil's fruit), it's meant to apply to any and all sins.
When the women journey through the Valley of Humiliation, Mercy has a much easier experience than many (like Christian, who had to literally fight his way through). Bunyan uses an allusion to the Bible's Song of Songs to illustrate how the Valley of Humiliation is a place of heartfelt contemplation for Mercy:
Then said Mercy, I think I am as well in this Valley as I have been anywhere else in all our Journey, the place methinks suits with my spirit. [...] Here one may think, and break at heart, and melt in one's spirit, until one's eyes become like the Fishpools of Heshbon.
The phrase "Fishpools of Heshbon" is found in Song of Songs 7:4, "Thine eyes [are] like the fishpools in Heshbon," where a bridegroom is describing his bride's beauties. The simile probably just means that the beloved's eyes are clear and brimming with life, like certain ponds found in ancient Israel. By describing a humble pilgrim's eyes this way, alongside a heart that "breaks" and a spirit that "melts," Mercy seems to mean that here in the Valley of Humiliation, a pilgrim is able to consider and weep over God's love until her eyes shine and overflow with grateful tears. Though heartbreak and weeping don't sound very inviting, to Bunyan's audience such emotional displays were signs of a redeemed heart that knows where it stands before God, as Mercy clearly does. That's why the Valley "suits with [her] spirit," or appeals to her.
Because the Song of Songs has often been interpreted as an allegory of the love between Christ and his people, this allusion also subtly underscores the value of allegory, namely that even ordinary Christians, like Mercy, should delight in allegorical images.
When Christiana and her fellow travelers grow weary and stop to rest at Gaius's Inn, it turns out that Gaius knows of Christiana's late husband and his family. His greeting makes multiple allusions, including an allusion to the first Christians recorded in the Bible:
Then said Gaius, Is this Christian’s Wife? and are these Christian’s Children? I knew your Husband’s Father, yea, also his Father’s Father. Many have been good of this stock, their Ancestors dwelt first at Antioch.
On one level of the story, Gaius speaks as though he knew Christian's literal ancestors, but on an allegorical level, he's referring to a verse in the Book of Acts, chapter 11, "And the disciples were called Christians first at Antioch." In other words, the Christian "family" originally hails from Antioch. On one hand, this is a slightly whimsical allusion that's meant to prompt a knowing chuckle from readers, but it's also Bunyan's way of saying that, indeed, those earliest Christians named in the Bible are the spiritual forebears of any Christian reading this book.
When Gaius elaborates on the "Christian" family ancestry, he names a veritable catalogue of biblical and historical Christians:
Stephen that was one of the first of the Family from whence your Husband sprang, was knocked o’ the head with Stones. James, another of this Generation, was slain with the edge of the Sword. To say nothing of Paul and Peter, men antiently of the Family from whence your Husband came, there was Ignatius who was cast to the Lions [...] and Polycarp that played the man in the Fire.
Bunyan's original audience would have quickly recognized these names, much as they would have spotted the allusion to Acts in the first quote. The first recorded Christian martyr, Stephen, was stoned to death in Acts 7; James, Paul, and Peter were each apostles who were killed for their faith; Ignatius of Antioch and Polycarp of Smyrna were both martyred during the second century C.E. Readers would know that these men weren't biological relatives of each other and that it's even less likely that they'd be direct ancestors of anyone today—thus highlighting the allegorical sense that these martyrs are all part of the extended "family" of faithful Christians, both in the book and among pious readers as well. Noting these formidable Christians' perseverance while facing brutal deaths isn't meant to be morbid, then, but a way of inspiring contemporary readers that even if they don't face similar deaths themselves, they share with the martyrs a faith worth dying for.
When Christiana's group of pilgrims passes through the Enchanted Ground, Bunyan uses a biblical allusion to show that faith is indispensable to their journey:
Now they had not gone far, but a great Mist and a Darkness fell upon them all, so that they could scarce for a great while see the one [or] the other. Wherefore they were forced for some time to feel for one another by Words, for they walked not by Sight.
"[T]hey walked not by Sight" is an allusion to 2 Corinthians 5:7, "For we walk by faith, not by sight." At this point in the New Testament letter, the apostle Paul exhorts Christians regarding the hope of eternal life. Even while Christians live life in their earthly bodies and are thus "absent from the Lord" in heaven, Paul explains in the preceding verse, they can be confident, knowing that someday they will enjoy eternal life with the Lord. They have this confidence "by faith" and not because they can see it with their eyes.
This allusion is a particularly apt one for Bunyan to insert while describing the group's journey through the Enchanted Ground, a place whose atmosphere tends to lull pilgrims asleep unless they make a concerted effort to stay alert. When Christiana's group passes through, the atmosphere is especially marked by a disorienting mist and darkness; they cannot rely on their sense of sight to find their way. Instead, the pilgrims keep talking in order to keep track of each other's locations as they go. By encouraging one another verbally, the pilgrims are able to ensure that nobody strays from the path and that everyone stays focused on their destination, even though they can't navigate by sight and be visibly assured that they're headed the right way. The struggle through the Enchanted Ground's mist and darkness also serves as a metaphor for the journey through earthly life, where the Christian hopes for eternal life with God even though that promise isn't accessible to the natural senses. As the Bible verse states, Christian confidence comes not from "sight," but from "faith"—and, Bunyan implies, faith in one's ultimate destination is maintained through mutual encouragement in a shared journey, not through wandering in solitude (a solitude that, implicitly, would tend to lead one astray).