The official interpretation is that the "enemies" must be set, by the GM, when the spell list is first learned. And once learned, the list of enemies will not change.
For example, if a Ranger were to gain this list somehow, the Enemies might be set as Orcs, Goblins, and Kobolds. And that is what it would remain.
For a Paladin of Reaan, the enemies list would be Priests/Paladins of Scalu, Demons, Undead, and creatures of the Unlife. And that list would never change.
Trying to use it against "anybody who means me harm at any time" makes the list way too powerful and I would strongly recommend and urge any and all GMs to not allow that.