Monolatrism

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, for example The Merchant's Prologue and Tale suggests that Chaucer believed in the existence of the Roman god Pluto and the goddess Prosperina though 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.

The Bible
The Bible is contradictory, some passages are best interpreted as implying that many gods exist.

Old Testament
In the Old Testament these sections include Exodus where the gods of the Egyptians are mentioned, also Pharaoh’s sorcerers can turn a staff into a snake. (Exodus 7) "Thou shalt have no other gods before Me" (Exodus) 20-3 can suggest that other gods exist, Psalm 86-8 reads, “Among the gods there is none like unto thee, O Lord; (…)”. Isaiah 44-6 says, “(…) beside me there is no god".

New Testament
Paul of Tarsus or whoever else wrote the epistles is contradictory. The 1st Epistle to the Corinthians suggests that there is no god but one. (1 Corinthians 8-4), despite this (2 Corinthians 4-4) refers to the god of this world which probably means the Devil.