Monotheism is the belief in a single god. It is usually contrasted with polytheism, belief in many gods. The world's major monotheistic religions (in chronological order of founding) include Judaism, Christianity and Islam.
"You shall have no other gods before me."
Although the most famous statement in the Abrahamic religions of monotheism is "Thou shalt have no other gods before me", the commandment itself implies other gods exist! This is one instance of the polytheistic origins of the Abrahamic religions.
"That is Allah, your Lord; there is no deity except Him, the Creator of all things, so worship Him. And He is Disposer of all things."
"[The other Gods of polytheism] —LAUGHED themselves to death once on a time! That took place when the unGodliest utterance came from a God himself—the utterance: 'There is but one God! Thou shalt have no other Gods before me!'"
We do not observe polytheism
- "God must be one. Otherwise we would see great differences and competition between the gods if there were one -- Alone."
Many religions claim that there is indeed competition between Gods, but the evidence for polytheism is no stronger than monotheism.