Thermopod dies, its persist ability triggers, returning it from your graveyard to the battlefield with a -1/-1 counter on it.
Activate
Thermopod by sacrificing
Sin.
When
Sin dies, it triggers twice.
Resolve the first trigger, returning
Sin from your graveyard to the battlefield with a -1/-1 counter on it. As
Sin enters, remove the -1/-1 counter from
Thermopod, causing
Sin to enter with a +1/+1 counter on it instead.
Resolve the second
Sin trigger, putting the counters it had on it when it was sacrificed onto a creature you control and failing to shuffle it into your library as it's no longer in your graveyard.