Zealous Conscripts and Verduous Gearhulk do not have a -1/-1 counter on them.
Steps
Activate Cauldron of Souls by tapping it, giving Verdurous Gearhulk and Zealous Conscripts persist until end of turn.
Activate Altar of Dementia by sacrificing Zealous Conscripts.
When Zealous Conscripts dies, its persist ability triggers, returning itself from your graveyard to the battlefield with a -1/-1 counter on it.
When Zealous Conscripts enters the battlefield, it triggers, untapping Cauldron of Souls.
Activate Altar of Dementia by sacrificing Verdurous Gearhulk.
When Verdurous Gearhulk dies, its persist ability triggers, returning itself from your graveyard to the battlefield with a -1/-1 counter on it.
When Verdurous Gearhulk enters the battlefield, it triggers, putting a +1/+1 counter on itself and Zealous Conscripts, and two +1/+1 counters on any other creature(s) you control.
Resolve both Altar of Dementia abilities, causing target player to mill cards equal to Verduous Gearhulk and Zealous Conscripts' power.
Repeat.
Results
Infinite +1/+1 counters on all other creatures you control.