When Enduring Bondwarden dies, it and Athreos' third ability trigger.
Resolve the Enduring Bondwarden trigger, putting all counters from it onto another nontoken creature you control.
Resolve the Athreos trigger, returning Enduring Bondwarden from your graveyard to the battlefield.
When Enduring Bondwarden enters the battlefield, its backup ability triggers, putting a +1/+1 counter on the creature who had a coin counter placed on it, and giving it the printed ability until end of turn.
Activate Ashnod's Altar by sacrificing the nontoken creature with a coin counter on it, adding ({C} magic symbol) ({C} magic symbol) .
When the creature dies, it and Athreos' third ability trigger.
Resolve the creature's trigger, putting all counters from it onto Enduring Bondwarden .
Resolve the Athreos trigger, returning the creature from your graveyard to the battlefield.
Repeat.
Results
Infinite +1/+1 counters on target creature you control.