Holy is one of the basic elemental attributes found in Castlevania. The Vampire Killer and the Claimh Solais inflict Holy damage, the Vampire Killer is especially potent against vampires (and all related to them), Dark element creatures are weak against this element.
Holy based attacks and weapons are very effective on the undead and demons, which makes them invaluable because they are effective on most enemies in the game. There are very few enemies that aren't weak to holy damage, and even less that are resistant to it. Enemies that resist holy based damage would be most hero-turn-villian bosses such as Richter Belmont, Julius Belmont, Maria Renard and Albus, as well as disguised creatures like Amalaric Snipers, White Fomor or the Nuculais. Usually, holy weapons tend to glow with power (eg. Vampire Killer or Claimh Solais), however, some holy weapons like Tome of Arms X don't glow at all and look like ordinary weapons.