This is the first post in a series which will deal with myths and piecing out the truths contained within.
Crosses will repel vampires and certain undead.
A cross itself has no special powers to repel the undead, this is one of the most dangerous myths spread by Hollywood. You can't just put two pieces of wood together perpendicularly and expect to ward off the fiercest predators of the night. A crucifix on the other hand is a very powerful tool that can be used by the faithful for protection.
The difference between the two is that a crucifix is the embodiment of Christ's sacrifice. It has a depiction of the martyred son of god while a cross is just two lines running across each other at a ninety degree angle.
Now with that said, a blessed cross, one made of pure wood and properly sanctified will work in the place of a crucifix, however it depends mostly on the faith of the wielder.
Anyone can use a cross/crucifix.
This is true in theory, any person can use a holy symbol to repel a vampire. However the efficacy of the repelling relies on the faith of the person. A Muslim will have no more luck with a crucifix than they would with a a Star of David.
While the metaphysics are not fully understood, the truer an individual's faith the more effective the implements they wield. It must also be the right implement that is used. To this end a hunter must be at peace with themselves and have accepted a belief structure.
In the 1999 film The Mummy, Beni is threatened by the undead and attempts to use many of them to protect himself.
Beni has no real faith and is only saved by his usefulness of knowing a language allowing the Mummy to enslave him. Attempting to use holy symbols en-mass with no real belief in them will 99% of the time result in death and not a fist full of treasure.
A Crucifix will only work on a Christian vampire.
This myth takes a truth and twists it slightly. Many vampires still hold true to their own religious beliefs even after death. However immortality disconnects the majority of Blood suckers from their spiritual connection over time. Interviews with the vampires and studies by psychologists have found that after a few decades of their own ageless splendor and hunting those the very people they were once a part of leads the vampires to change their own morality and belief system to preserve their psyche.
Therefore a younger vampire is more likely to retain their religious beliefs than an older one. As a vampire ages they increase in strength and holy symbols, while still being able to repel and protect, will not have nearly the spectacular effect that they will on a younger brood.
In the field it is the age of the vampire and the belief of the wielder that determines how well a symbol works. A Christian wielding a Crucifix will have as much effect on a Muslim Vampire as they would a Buddhist Vampire as they would a Christian Vampire. Once more we must stress that it is the person and their faith that repels and not just the holy symbol.