The use of vessels by gods can be likened to the use of Accumulation by demons to reach the power of a demon lord. It is not a sustainable option and it cannot compare to the real thing. Vessels can't use the full power of a god. They will die if it is tried so all those vessels will probably die too.
Even Stelios's zealots will not be able to hold back the demon king for long. They both have Authority but Stelios's vessels will break first before the demon king and they will break easily too. The vessels of the other gods are sourly lacking. They wouldn't be of any use at all. So the liberation army is being sent to their deaths.