Basically what it says on the tin. What is the latest possible POD to preserve or revive Greco-Roman Polytheism as a significant religious and philosophical force in the Western World?
This doesn't necessarily mean that Christianity or Monotheism has to be avoided and I allow for perhaps a Chinese style situation where for example Buddhism, Daosim or Confucism had state sanction while the Ancient local cults and practices survive while the powers at be alternate between persecuting one and supporting another at various times and the end result doesn't necessarily have to look anything like the Classical Polytheism we know. But all the basic hallmarks of the religion i.e functioning temples, sacrifices being performed, the basic moral precepts, academies and institutions not to mention at least a sizeable minority or worshippers must survive.
Brownie points for anyone who can revive polytheism after the death of Emperor Julian.