Children of Men explores the themes of hope and faith in the face of overwhelming futility and despair. The film's source, the novel The Children of Men by P. D. James, describes what happens when society is unable to reproduce, using male infertility to explain this problem. In the novel, it is made clear that hope depends on future generations. James writes, "It was reasonable to struggle, to suffer, perhaps even to die, for a more just, a more compassionate society, but not in a world with no future where, all too soon, the very words 'justice,' 'compassion,' 'society,’ 'struggle,' 'evil,' would be unheard echoes on an empty air."
The film switches the infertility from male to female, but never explains its cause: environmental destruction and divine punishment are considered. This unanswered question (and others in the film) have been attributed to Cuarón's dislike for expository film: "There's a kind of cinema I detest, which is a cinema that is about exposition and explanations ... It's become now what I call a medium for lazy readers ... Cinema is a hostage of narrative. And I'm very good at narrative as a hostage of cinema." Cuarón's disdain for back-story and exposition led him to use the concept of female infertility as a "metaphor for the fading sense of hope". The "almost mythical" Human Project is turned into a "metaphor for the possibility of the evolution of the human spirit, the evolution of human understanding." Without dictating how the audience should feel by the end of the film, Cuarón encourages viewers to come to their own conclusions about the sense of hope depicted in the final scenes: "We wanted the end to be a glimpse of a possibility of hope, for the audience to invest their own sense of hope into that ending. So if you're a hopeful person you'll see a lot of hope, and if you're a bleak person you'll see a complete hopelessness at the end."
Children of Men takes an unconventional approach to the modern action film, using a documentary, newsreel style. Film critics Michael Rowin, Jason Guerrasio and Ethan Alter observe the film's underlying touchstone of immigration. Alter notes that the film "makes a potent case against the anti-immigrant sentiment" popular in modern societies like the United Kingdom and the United States, with Guerrasio describing the film as "a complex meditation on the politics of today".
For Alter and other critics, the structural support and impetus for the contemporary references rests upon the visual nature of the film's exposition, occurring in the form of imagery as opposed to conventional dialogue. Visually, the refugee camps in the film intentionally evoke Abu Ghraib prison, Guantanamo Bay detention camp, and The Maze. Other popular images appear, such as a sign over the refugee camp reading "Homeland Security". The similarity between the hellish, cinéma vérité stylized battle scenes of the film and current news and documentary coverage of the Iraq War, is noted by film critic Manohla Dargis, describing Cuarón's fictional landscapes as "war zones of extraordinary plausibility".
In the film, refugees are "hunted down like cockroaches", rounded up and put into cages and camps, and even shot, leading film critics like Chris Smith and Claudia Puig to observe symbolic "overtones" and images of the Holocaust. This theme is reinforced in the scene where an elderly refugee woman speaking German is seen detained in a cage, and in the scene where British Homeland Security strips and beats illegal immigrants; a song by The Libertines, "Arbeit Macht Frei", plays in the background. "The visual allusions to the Nazi roundups are unnerving," writes Richard A. Blake. "It shows what people can become when the government orchestrates their fears for its own advantage."
Cuarón explains how he uses this imagery to propagate the theme by cross-referencing fictional and futuristic events with real, contemporary, or historical incidents and beliefs:
They exit the Russian apartments, and the next shot you see is this woman wailing, holding the body of her son in her arms. This was a reference to a real photograph of a woman holding the body of her son in the Balkans, crying with the corpse of her son. It's very obvious that when the photographer captured that photograph, he was referencing La Pietà, the Michelangelo sculpture of Mary holding the corpse of Jesus. So: We have a reference to something that really happened, in the Balkans, which is itself a reference to the Michelangelo sculpture. At the same time, we use the sculpture of David early on, which is also by Michelangelo, and we have of course the whole reference to the Nativity. And so everything was referencing and cross-referencing, as much as we could.
In the closing credits, the Sanskrit words "Shantih Shantih Shantih" appear as end titles. Writer and film critic Laura Eldred of the University of North Carolina at Chapel Hill observes that Children of Men is "full of tidbits that call out to the educated viewer". During a visit to his house by Theo and Kee, Jasper says "Shanti, shanti, shanti." Eldred notes that the "shanti" used in the film is also found at the end of an Upanishad and in the final line of T. S. Eliot's poem The Waste Land, a work Eldred describes as "devoted to contemplating a world emptied of fertility: a world on its last, teetering legs". "Shanti" is also a common beginning and ending to all Hindu prayers, and means "peace," referencing the invocation of divine intervention and rebirth through an end to violence.
Like Virgil's Aeneid, Dante's Divine Comedy, and Chaucer's Canterbury Tales, the crux of the journey in Children of Men lies in what is uncovered along the path rather than the terminus itself. Theo's heroic journey to the south coast mirrors his personal quest for "self-awareness", a journey that takes Theo from "despair to hope".
According to Cuarón, the title of P. D. James' book (The Children of Men) is a Catholic allegory derived from a passage of scripture in the Bible. (Psalm 90 (89):3 of the KJV: "Thou turnest man to destruction; and sayest, Return, ye children of men.") James refers to her story as a "Christian fable" while Cuarón describes it as "almost like a look at Christianity": "I didn't want to shy away from the spiritual archetypes," Cuarón told Filmmaker Magazine. "But I wasn't interested in dealing with dogma."
|“||Ms. James's nativity story is, in Mr. Cuarón's version, set against the image of a prisoner in an orange smock with a black bag on his head, arms stretched out as if on a cross.||”|
—Manohla Dargis, 
This divergence from the original was criticised by some, including Anthony Sacramone of First Things, who called the film "an act of vandalism", noting the irony of how Cuarón had removed religion from P.D. James' fable, in which morally sterile nihilism is overcome by Christianity.
The film has been noted for its use of Christian symbolism; for example, British terrorists named "Fishes" protect the rights of refugees. Opening on Christmas Day in the United States, critics compared the characters of Theo and Kee with Joseph and Mary, calling the film a "modern-day Nativity story". Kee's pregnancy is revealed to Theo in a barn, alluding to the manger of the Nativity scene, when Theo asks Kee who the father of the baby is she jokingly states she is a virgin, and when other characters discover Kee and her baby, they respond with "Jesus Christ" or the sign of the cross. Also the Archangel Gabriel (among others divinities) is invoked in the bus scene.
To highlight these spiritual themes, Cuarón commissioned a 15-minute piece by British composer John Tavener, a member of the Eastern Orthodox Church whose work resonates with the themes of "motherhood, birth, rebirth, and redemption in the eyes of God." Calling his score a "musical and spiritual reaction to Alfonso's film", snippets of Tavener's "Fragments of a Prayer" contain lyrics in Latin, German and Sanskrit sung by mezzo-soprano, Sarah Connolly. Words like "mata" (mother), "pahi mam" (protect me), "avatara" (saviour), and "alleluia" appear throughout the film.