Board Thread:Spoilers!/@comment-24674051-20150720164321/@comment-25926288-20151001140131

Chameleon-Guy wrote: I also think that's the reason Regina is the Savior, because right now she is the only one who can use light magic. Well, I don't think Emma can't use light magic. They never gave us a reason stating otherwise. It would be weird for her to lose her magic just to gain Dark magic.

I think that's why she can try to convince them that she can be the Savior, but she isn't the Savior. Emma will always be the Savior, unless they changed the requirement. Emma still has the most powerful light magic, but Arthur and co don't know that. That's probably why the Furies or something go after her because she doesn't fit the bill for the Savior.