Hello.
So I like the idea of your deck. I also have an Athreos deck. The thing to remember is that it works best if you can get a lot of cheep creatures out that can either hit for a lot on the field or as they die. In the spirit of the deck, and because your deck also has red, I suggest playing
Vexing Devil. It goes perfectly with Athreos, and is a wonderful creature in general.
In addition, in case your opponent chooses to leave some key creatures
dead, I would bring add some cards that bring creatures back.
Return to the Ranks,
Whip of Erebos,
Immortal Servitude, and things like that could be a huge help.
The last card I want to suggest is
Xathrid Necromancer. It goes extremely well with Athreos, and the large number of Human creature cards you have.
I think your deck, while needing a bit of work, is on its way to be a strong deck. Nice job.