The end of East of Eden can literally be summed up in one word: timshel. What is timshel, you ask? Is it something you find on the beach? Is it a type of pasta? Nope: it's a Hebrew word that Lee translates as Thou mayest, as in, You can if you want to.
But what is Hebrew doing in a book that takes place in Salinas? This is where the whole biblical aspect of East of Eden really takes center stage. You'll recall that Lee first mentions timshel when he is talking about what God says to Cain in the Book of Genesis after he turns down Cain's gift and Cain gets all mad about it. This is the passage Samuel reads the first go around:
"And the Lord said unto Cain, "Why art thou wroth? And why is they countenance fallen? If thou doest well, shalt thou not be accepted? And if thou doest not well, sin lieth at the door. And unto thee shall be his desire, and thou shalt rule over him." (22.4.29)
That's Genesis 4:6-7, and it's how the King James Bible translates the passage from what was originally Hebrew. If Shmoop were to translate this passage, it might go something like this:
"Cain, why are you so mad? If you get me a good gift, won't I accept it? And if you don't, you're inviting in sin. And it's gonna try to make you sin, and you're gonna rule over it."
But, as Lee points out later on, timshel shouldn't actually be translated as thou shalt, which is an outright command. Instead, timshel read as thou mayest implies a choice: so that last bit might run, "You might rule over sin, but it's up to you whether you do or not" (the him in the original translation is sin).
Okay, so now we know about timshel, but it's also Adam's last word before he dies (at least it wasn't something totally random like rosebud). He says it right after Lee gets him to give Cal his blessing, i.e. show that he loves him as a son and free him from the guilt of "killing" his brother. It's a moment where things are at a crossroads for Cal: he could go on hating himself for what he has done and thinking that his dad died hating his guts, or he can be free and go on to break the Cain-Abel curse that seems to follow the Trask family around.
As Lee says to Adam, "Give him his chance" (55.3.52). Chance means just that: it's not totally inevitable that the Trasks are just going to keep producing fathers who pick favorites and siblings who want to kill each other. So when Lee tells Adam to say Cal's name and Adam says "Timshel!" instead, it tells us that Cal has to decide whether or not he continues the curse, regardless of Adam's blessing. You might say that the message here is that we have to be held accountable for our actions, because nothing makes us do them—not even a biblical curse.