From Diablo Wiki
A few monsters deal holy damage, and it is mitigated by the Arcane resistance.
Holy Damage Effects
Holy damage in Diablo 3 is simply another type of damage, dealing full damage at the time of impact and manifesting purely as another color of elemental. This is a change from Diablo 2, where Holy damage came only from a few Paladin skills, only affected the Undead (who it dealt bonus damage to), and healed other characters and minions if it struck them.
The Reaper of Souls expansion may return some special properties to Holy Damage, especially in the form of Crusader skills.