Rachel Bilson was born and raised in Los Angeles, California.
Bilson's father is Jewish and her mother is Catholic. Technically one is only a Jew if one's mother is a Jew, so by orthodox standards she missed that boat. And even though she went to Catholic schools growing up, there's no indication that she considers herself Catholic either.
I couldn't find anything about her going to church, talking about Jesus, meditating, praying, or even feeling blessed. She doesn't appear to be an atheist either. The closest thing to religion I could get out of her was her obsession with fashion. So we'll have to give her the good old "non-religious" tag until further notice.
The fact that Bilson's father was a producer and director in Hollywood, and her mother was a sex therapist, sure doesn't scream conservative. And she's not.
In a sort of gross paparazzi video of Bilson walking her dogs, the filmer asks if she's voting for John McCain or Barack Obama, and she politely says Obama. She was also spotted stopping to sign a gay marriage rights petition in Los Angeles in 2010.
But she's definitely not going out of her way to talk politics. It's possible that she's liberal by default–her parents were probably liberal, and growing up in Hollywood, most of her peers probably are too. But she's not passionate enough about it, at least in public, to make it seem like she's really dedicated to liberal causes.