The main theme revolves around the timeless battle between good and evil. This struggle can manifest between two people or internally within one person grappling with their moral decisions. God has granted humans free will, allowing them to choose to embrace goodness over evil if they wish.

This concept is rooted in the biblical tale of Cain and Abel from Genesis, chapter 4, verses 1 to 16. Cain and Abel were the first sons of Adam and Eve. While Cain worked the land, Abel tended to sheep. When they presented offerings to God, He preferred Abel's sacrifice of the firstlings of his flock over Cain’s crop offerings. Enraged by this, Cain killed his brother. As a consequence, God condemned Cain to wander the earth. Fearing for his life, Cain was in despair, but God marked him to protect him from being slain. Cain eventually settled in the land of Nod, east of Eden.

This Cain and Abel narrative is mirrored in the novel through the initials of the characters. Charles and Adam Trask represent Cain and Abel, respectively. This is shown when their father, Cyrus, rejects Charles’s gift of a knife but accepts Adam’s puppy offering. Charles’s response echoes Cain's fury; he becomes enraged and viciously attacks his brother. If he had found Adam with a weapon, he would have killed him. Charles also has a scar on his forehead, reminiscent of the mark God gave Cain.

The allegory continues with the next generation of Trasks. Caleb inherits Cain's legacy, while Aron symbolizes Abel's innocence. Just as Charles resented and sought revenge whenever Adam surpassed him, Caleb envies Aron’s greater popularity. Caleb undermines Aron by playing tricks, such as when he gave Abra a dead rabbit that Aron had intended as a gift.

The Cain and Abel pattern continues into Caleb and Aron’s teenage years. Caleb’s monetary gift to his father is rejected, whereas his father praises Aron’s academic achievements and aspirations to attend college. Hurt by his father's rejection, Caleb symbolically destroys Aron by revealing the truth about their mother Cathy, leading Aron to enlist in the army, where he ultimately dies in battle.

The symbolism of initials extends to Abra and Cathy, representing good and evil, respectively, and to some degree, to Cyrus and his wife Alice. Like Charles, Cathy bears a scar on her forehead, indicating her alignment with Cain’s malevolence.

Steinbeck emphasizes the Cain and Abel story through Lee's discussion with Samuel and Adam about his interpretation. The focus is on the Hebrew word "timshel." This word appears in the passage where God tells Cain, "thou shalt" (timshel) rule over sin. An alternative translation uses "Do thou" instead of "thou shalt." Lee is intrigued by the distinction between these translations. "Thou shalt" suggests a promise that Cain would defeat sin, a promise not fulfilled by his descendants. In contrast, "Do thou" implies a command. Lee consults a group of elderly Chinese scholars in San Francisco, who learn Hebrew specifically for this purpose. They suggest a new translation for timshel: "thou mayest." To Lee, this is profoundly meaningful. It indicates that humans have the freedom to make choices. They can opt to overcome sin or not. Nothing is predestined.

This theme is mainly explored through the character of Caleb. As a teenager, Caleb is keenly aware of the Cain legacy he inherited from Cathy and struggles against it, sometimes unsuccessfully. Even as a child, he prayed to God to be like Aron: “Don’t make me mean. I don’t want to be.” When he grows older, Lee tries to assure him that inheriting part of his mother’s nature...