User blog comment:LastationLover5000/Gods or Spirits/@comment-3061966-20130715222757/@comment-3061966-20130716020006

In the FTverse, history has clearly been manipulated. No one knows about the Dragon civil war beyond a select bunch, the truth of Zeref, anything of substance about Acnologia's origins beyond a few people, etc. So, the name of a lost magic changing over time is not far fetched.

Aha, a name is just a name. Think about it, for a second. Let's say it's really a magic meant for slaying demons, and like I said,  Zeref has/had the power (supposedly) to raise armies of artificial demons according to Ultear. Amongst the Zeref-obessed dark mages, Zeref is their god -- we saw that during Erza's flashback of her Tower of Heaven days. He's the most powerful mage in history, so he's god-tier in terms of battle prowess. If God Slayer magic was meant to slay the demon armies created by Zeref, a feat akin to a god, then it starts to make some sense why the name came to be God Slayer magic. Besides, there's no spirits to go slay in Earth Land.