A town has a festival every year in the dead of winter. The festival of birds. The towns folk dress in feathery robes and beaked masks and dance and frolic in the belief that the changes of the seasons are controlled by the presence of birds. According to tradition, the festival tricks Spring into coming early because the birds have returned.
A cult's theology is binding on the god involved: it is possible for that god to believe (and see, just like Winston Smith in 1984) that other gods are mythical, or jumped-up demons, or failing older spirits, even while those gods actually exist and thrive and act in the cosmos, even while working against him.