As for Angels and magic. I see as simply being at a higher level and also more naturally used. Fire for example. I feel that only extraordinary humans can get to some kind of middle tier fire and that is still capable of destroying kingdoms. but Sacred and Hellfire are Top Tier fire. that is innate for the race but at a low 1% are able to utilize it. or human might be able to achieve such a flame if they had assistance from an outside source.

As for origins. It just depends how Angels came to be. Do ancient heroes become angels if they reach a certain level of divinity through their achievements. So maybe when they ascend to be angels maybe some lose their memory. Or... perhaps him losing his memory is part of a bigger plan from God. 

Those are my two cents anyways

Edit: Just noticed Black Alice's post. I have to agree on the lackluster power. I personally think that my summoner is a bit underpowered compared to your three's characters. I'll have to focus more on impressive summons and unique magical items to play with the big boys and girls.

Maybe make the sacred flame an actual living entity. Almost like a Guardian Angel for an Angel. Some kind of mystical life form that was gifted to Achilles from God.