Athreos has a dies trigger, so we look at what the card looked like when it was last on the battlefield to see if Athreos should trigger. When it was last on the battlefield, the manifested card was a creature, so Athreos will trigger (even if it's not a creature card while in the graveyard). It's face up while in the graveyard, so your opponent will know what the card is well before the trigger from Athreos resolves.
For instance, let's say you manifested a Shock face down. When that manifested card dies, Athreos saw it was a creature, so Athreos will trigger. When it's in the graveyard, it's face up, so everyone knows that it's a Shock. When the trigger resolves, the targeted opponent chooses to either pay 3 life or let you get the Shock back into your hand.
Nathan Long
Wizards.Com Boards NetRep