Monolatrism is the belief that many gods exist but believers should worship only one. Up till the 18th Century Christians believed or generally frequently that other gods existed but that Christians should worship only one. Wikipedia: The Merchant's Prologue and Tale suggests that Chaucer believed in the existence of the Roman god Pluto and the goddess Prosperina. As a Christian Chaucer did not worship them. Roman mosaics of the baptism of Jesus sometimes show the river god in the river Jordan. Romans believed that a river god is in every river.