My chosen benediction when I learn of someone's death is "sit ei terra levis." I chose it after I was informed — as an agnostic who just took it for granted — that "rest in peace" is specifically associated with Christian burials.

"Sit tibi terra levis" is a benediction for the dead from ancient Roman tombs. It means "may the earth lie lightly upon you". I've swapped the "tibi" for "ei" — "you" for "that person".