Holy damage treats all targets as having 60 armor, at 60AL you do the same damage as on skill description. Less than 60 & you do more damage on the target, more than 60AL & you do less damage than skill description.
(If there is somehow something that gives 100% armor penetration you will do ALOT more damage than what description says).
Armor & damage calc you can play around with it to see what happens.
Edit:
game mechanics page explains this also