East of Eden is a clear biblical allegory, repackaging stories and other excerpts from the Bible as tales that connect to American culture. The Bible itself is filled with allegories, many of which are stories told by Jesus or the prophets, all of whom use these narratives to teach some ethical or spiritual lesson. It may be more accurate to state that East of Eden presents these allegorical aspects in a new context, morphing themes from the Bible into a commentary on late 19th- and early 20th-century America.
Steinbeck assigns the characters in his novel "roles" within the biblical allegories they play out, sometimes stating these connections directly. Take, for example, the following passage from Chapter 15:
"Look, Samuel, I mean to make a garden of my land. Remember, my name is Adam. So far I've had no Eden, let alone been driven out."
"It's the best reason I ever heard for making a garden," Samuel exclaimed. He chuckled. "Where will the orchard be?"
Adam said, "I won't plant apples. That would be looking for accidents."
"What does Eve say to that? She has a say, you remember. And Eves delight in apples."
Note that in this passage, Adam is very clearly placing himself in the position of the biblical Adam, father of all mankind. Samuel places Cathy in the position of Eve, implying that Cathy will betray Adam somehow and bring about the "fall" of man.