IndestructibleIndestructible is a static ability.
A permanent with indestructible can’t be destroyed. Such permanents aren’t destroyed by lethal damage, and they ignore the state-based action that checks for lethal damage (see rule 704.5g).
Multiple instances of indestructible on the same permanent are redundant. (CR 702.12)
As long as your devotion to white and black is less than seven, Athreos isn't a creature.
At the beginning of your end step, put a coin
counterTo counter a spell or ability is to cancel it: remove it from the stack without resolving it. A counter on an object or player is instead a marker that changes characteristics or interacts with rules or abilities; for example, a +1/+1 counter modifies a creature’s power and toughness. (CR 701.6; 122.1) on another target creature.
Whenever a creature with a coin
counterTo counter a spell or ability is to cancel it: remove it from the stack without resolving it. A counter on an object or player is instead a marker that changes characteristics or interacts with rules or abilities; for example, a +1/+1 counter modifies a creature’s power and toughness. (CR 701.6; 122.1) on it
diesIs put into a graveyard from the battlefield. (CR 700.4) or is put into
exileTo exile an object, move it to the exile zone from wherever it is. See rule 406, “Exile.” (CR 701.13), return that card to the battlefield under your control.