There is no consistent definition of Deity that simultaneously includes Zeus, Loki, Hades, etc. while excluding Satan, Angels, Demons, etc.
To me, it seems that most Christians put other supernatural beings (especially The Devil) on such a high pedestal that they might as well be gods, making Christianity more of a monolatry (or, if the definition of deity is changed, then the ancient Greeks would be atheist).
For the purposes of this debate, I am ignoring the trinity.
If you can give me a consistent definition of deity that includes the polytheistic pantheons and excludes all non-God entities in Christianity, I'll happily concede the debate. But I will hold you to your definition.
Common definitions I expect to hear:
Deities are eternal, or omnipotent, or omniscient: then Zeus, Hades, etc. are not deities, and the Greeks are atheist.
Deities are what the religion says they are. This is a trivial definition, "X means what people say X means." It makes words like "theist," "atheist," "god," "deity," etc. meaningless. Points for ignosticism.
Deities have an independent will, and the Devil, angels, etc. are just will-less "robots" of God. This seems to go against traditional Christian theology (how can an angel fall without an independent will?). Also, are the immortal, willful souls of humans deities?
Note: I'm Jewish in upbringing and culture, since it reflects my knowledge and traditions, but my beliefs are more agnostic atheist.