There is a crucifix, a few cloves of garlic, a wooden stake, a hammer, a blob of Silly Putty, and a pocketknife. “You do realize these people aren’t vampires, right?†I say when Sam walks back in. “Yeah, but you never know. They’re probably crazy, like you said.†“And even if we were hunting vampires, what the hell is the Silly Putty for?†He shrugs. “Just want to be prepared.
He’s not feeling well,†Clary said, catching at Simon’s wrist. “We’re going.†“No,†Simon said. “No, I — I need to talk to him. To the Inquisitor." Robert reached into his jacket and drew out a crucifix. Clary stared in shock as he held it up between himself and Simon. “I speak to the Night’s Children Council representative, or to the head of the New York clan,†he said. “Not to any vampire who comes to knock at my door —“ Simon reached out and plucked the cross out of Robert’s hand. “Wrong religion,†he said.
Society expects man to be a passive social animal who believes like the People of the Field in "Jurgen" that "to do what you always have done" and "what is expected of you" are the twin rules of life. This, of course, is not true. The wanton crucifixion of impulses, the unnecessary blocking and frustration of the drives and urges, are an evil that reflects itself in sophistication, ennui and boredom, dissatisfaction, melancholy, fatigue, anxiety and neurosis.
What are all these?" Clary asked. "Vials of holy water, blessed knives, steel and silver blades," Jace said, piling the weapons on the floor beside him, "electrum wire - not much use at the moment but it's always good to have spares - silver bullets, charms of protetion, crucifixes, stars of David-" "Jesus," said Clary "I doubt he'd fit." "Jace." Clary was appalled.
The reason people use a crucifix against vampires is because vampires are allergic to bullshit.