Ur-priests cast divine spells.Special: The character must have no ability to cast divine spells.
Obviously, this is meant only for preexisting abilities, but strict reading of RAW* gives no such exception.
It's sort of like the Schrödinger's Dragon Disciple case.
(*Provided one accepts Complete Warrior and Complete Arcane's "if you no longer meet the requirements of a PrC you lose access to its class abilities" rule extends to other Prestige Classes.)