This novel belongs to the mythic category of the bildungsroman. It's mythic because the story is highly archetypal and the plot structure is one that every adult is intimately familiar with through their own experience. In other words, the character Billy represents the typical experience a young person has of life. It's kinda hard, but if you try to do the right thing, and you stay humble, everything falls into place. We see this because Billy is a cowboy in deep Western territory—not an easy life—and he is constantly under the threat of danger, constantly in risk of scarcity, constantly in survival mode. This is McCarthy's depiction of what life is like to teenagers. The Crossing is therefore a bildungsroman about the danger of arrogance and the value of social skills and humility in the game of life.
Basically, the kid makes the hero's quest, and his transformation from a child to an adult happens over the course of three hero cycles, three ventures into the unknown Mexican wilderness. The reader should know that Cormac McCarthy is famously skilled at dark, macabre poetic prose. But in this novel, he strips the gore way back to tell the story of a sensitive, noble character who finds a way of life that is not violent and gory like his surroundings. Therefore, Billy is a representation of moral virtue, whereas his brother represents the other path, the path of moral compromise. Why choose to behave with virtue and nobility? Because the other option makes you badly integrated with the real dangers of daily life. In other words, if you indulge your anger, let's say, perhaps some drunk cowboy will shoot you with a gun. Sometimes that happens.
The novel is simultaneously answering religious questions by ignoring them. The character doesn't have some elaborate religious life—rather, he has adopted a religious attitude toward life that makes him patient and curious. He treats every animal and person as if they are divine and worthy of life. This makes him a superhero, because he learns wisdom through experience, without having to pay for it by suffering. When he does suffer a brutal attack, the universe sends a helper to get him back in the game. He has already overcome many of life's most difficult challenges, by being humble. In this way, he represents the Christ character, but actually, the philosophy expressed in the novel has echoes of Zen Buddhism.
To interpret the book as a Siddhartha type Zen Buddhist story is simple. Notice that the character is noble because he treats every animal with non-violence, and he loves the animals—and people for that matter. Instead of arguing with reality like his brother, he just learns what there is to learn about the earth and contemplates. His perfect perception leads him to special insights and a peaceful mind, even in his difficult terrain. All these concepts are Zen ideas in disguise. In this way, he is like the Buddha, or like Jesus, but as the ending of the novel indicates, the character is not perfect. Yes, he suffers, but ultimately, he is spared from the wrath of McCarthy's god-like imagination. To quote another McCarthy novel, Billy spares himself by mindfulness from "a hell more horrible yet than the brimstone land of Christian reckoning."
