The 370-ish-year-old cross that hangs outside Carlisle's office belonged to his father – a 17th-century pastor who was intolerant not only of Catholics, but also of werewolves, witches, and vampires. He led hunts for all of these creatures, as did his son. Carlisle kept the cross as a family heirloom of sorts after he transformed. Is that the only reason? We don't think so. Carlisle's dad hated "evil" beings, and Carlisle, like his dad, abhorred evil acts. Now as a vampire, a creature to whom evil comes instinctively – he works hard not to give in to his primal nature. It's our theory that the Carlisle's cross serves to remind him that although he may be a vampire, he can resist his inner-urge to kill humans. The cross is also funny in an ironic way, as Edward notes, as certain versions of vampire myths say that crosses keep vampires away.