As far as I know, the word "Akuma" does not necessarily mean "the christian Devil" as in "Satan" or "Lucifer", but is more of an umbrella term for "evil spirit", under which Satan belongs too. "Akuma" seems to be non-specific.
Satan has its own name in japanese (サタン).
Check this japanese wiki entry.Also, the word used to describe Dracula is "Maou (魔王)" which means "Demon King." When translating the Satan's Ring, for instance, the translator(s) misunderstood this word as referring to THE christian Devil, and not to Dracula. This caused the item to be called "Satan's Ring" instead of "Demon King's/Dark Lord's Ring" -- which caused quite a big misunderstanding of the plot amongst my country's fanbase, because there is no "Satan" in Castlevania. Dracula is the big shot.
By what I have researched, this word is more appropriate to refer to THE Devil of christianity as it is an specific term for the highest "evil being" of a belief system.
Here this is mentioned.
But I think only a true japanese speaker/native can elucidade this beyond doubt.