The crux of Good Omens by Neil Gaiman and Terry Pratchett lies in its humorous and satirical take on the impending apocalypse, focusing on the unlikely partnership between an angel and a demon who work together to prevent the end of the world. The novel blends fantasy, comedy, and insightful commentary on humanity, good and evil, and the nature of free will.

Key Themes and Crux:

1. The Apocalypse is Coming:
   The novel is set against the backdrop of a prophesied Armageddon. The Antichrist, a child, is destined to bring about the end of the world when he turns 11. Aziraphale, the angel, and Crowley, the demon, have been on Earth long enough to become fond of it and its inhabitants. Both are reluctantly committed to their celestial duties but are reluctant to let the world end. They team up to try to prevent the apocalypse.

2. Aziraphale and Crowley’s Unlikely Partnership:
   The two main characters, Aziraphale and Crowley, represent good and evil but are deeply flawed and more human than divine. Aziraphale is a bookish, rule-abiding angel, while Crowley is a smooth-talking, mischievous demon. Over centuries, they have become friends and have come to enjoy Earth, which makes them hesitant to see it destroyed. Their collaboration to stop the apocalypse is central to the plot and forms much of the humor and satire.

3. The Mistaken Identity of the Antichrist:
   The Antichrist, who is supposed to bring the apocalypse, is accidentally switched at birth. The real Antichrist, named Adam, grows up unaware of his true nature as a normal boy. Meanwhile, the forces of good and evil struggle to locate him and ensure that the apocalypse proceeds as foretold. The mix-up leads to a series of comedic and chaotic events.

4. The Four Horsemen of the Apocalypse:
   The Horsemen—War, Famine, Pollution (instead of Pestilence), and Death—are set to ride and bring about the end of days. Their arrival is a humorous yet ominous part of the novel, with each Horseman embodying modern concerns and societal problems. The Horsemen serve as both a literal and symbolic representation of the world's problems.

5. Themes of Free Will vs. Destiny:
   Good Omens explores the tension between destiny and free will. While the prophecy suggests that the Antichrist must fulfill his role in bringing about the end of the world, the characters question whether they can change the course of events. The novel ultimately suggests that free will and individual choices can have a profound impact on the future.

6. Human Characters and the Prophecy:
   Human characters, such as the witch Anathema Device and her ancestor’s prophetic book, the group of children linked to the Antichrist, and the various quirky individuals who come into contact with the celestial beings, all influence the unfolding of the events. These characters play an important role in unraveling the mystery and preventing the apocalypse.

7. Humor and Satire:
   The novel is filled with witty dialogue, absurd scenarios, and social commentary. It critiques bureaucracy, religion, human nature, and the concept of good versus evil. The blend of humor and deep themes makes the novel both entertaining and thought-provoking.

Core Message:

The core message of Good Omens is a comedic exploration of the battle between good and evil, the futility of rigid prophecy, and the complexity of human nature. The novel suggests that despite the overwhelming forces of destiny and the celestial hierarchy, individuals—whether they are angels, demons, or humans—can change the course of events. At its heart, the book champions the unpredictability of life, the importance of free will, and the idea that good and evil aren’t always as clear-cut as they seem. The humor and wit make these serious themes more accessible and enjoyable.