I'm trying to understand your question. I think the easiest way to rule it is, if it's not a valid choice for Deific obedience then it isn't a valid choice for Evangelist.
Since Ragathiel is an Empyreal Lord you have to take Celestial Obedience in order to gain the associated boons.
If a DM wants to make an exception they certainly could and I don't see there being a problem with it.