Some kids learn about sex from their friends. Some learn about it from the Internet. But if you're Marcelo, you learn about it from the rabbi with whom you hang out on the regular, even though you're Catholic. Hey, it makes the story of Adam and Eve a lot more interesting, right?
Rabbi Heschel is Aurora's friend, so when Marcelo gets all interested in religion, Aurora introduces them. And for that, she wins the Coolest Catholic Mom Ever award (okay, we made that up, but if there were one, she'd totally win it.) Little does Rabbi Heschel know that she'll eventually be called on to (a) explain why Wendell talks about sex in such an obnoxious way, and (b) encourage Marcelo to do the right thing and save Ixtel.
In fact, she doesn't ever know either of those things directly, but she has her suspicions. After all, Marcelo wasn't asking such complicated questions until he started working at the law firm. Rabbi Heschel's no dummy; she knows the real world has thrown Marcelo some major curve balls. So when she says, "All of our inclinations, even the sexual ones, are good when we are in Eden—that is, when we walk with God and all our actions, words, and thoughts seek to follow his will. But man can choose to be separate from God, and in this separateness he creates evil by imagining ways to use what is good in ways that hurt him or others, and then acting upon what he imagines" (12.52), she's giving Marcelo some serious Words to Live By.
When you look at that quote in terms of sex, she's basically saying that yes, sex is meant to be a beautiful thing, but people like Wendell can use it for evil. But she's also talking about Arturo—indirectly, of course. Not only has he hurt others by fooling around with Jasmine, he's seriously hurt Ixtel by defending Vidromek. The latter has nothing to do with sex, of course, but it has a lot to do with Arturo going into law to help people and coming out defending the corporations who hurt them.
Rabbi Heschel is Marcelo's mentor, and she's the one adult in the book who seems to really see things clearly. After all, she's able to say, "I think we, and I mean all of us, every single one of us who's in the religion business, have messed things up royally" (26.52), and still find a reason to clean up the temple parking lot.
Marcelo sees the world in black and white, but Rabbi Heschel sees it all in shades of gray. Every kid who's going to change the world—whether the world at large, or one individual's world, like Marcelo does for Ixtel—needs a Rabbi Heschel. And we're not just saying that because we want to ride in a Volkswagen named Habakkuk, even though that would be pretty sweet.