Monotheism is the doctrine or belief that there is only one God.
The earliest forms of monotheism can be found in the Middle East under Zoroastrianism, the cult of Aten in Egypt and early Hinduism in India.
Zoroastrianism was based in Persia and originated around 1500 BCE. It was founded by the prophet Zarathushtra and was strictly monotheistic in their worship of Ahura Mazda. Zoroastrianism also gave birth to the doctrines of Satan, sin, angels and demons and heaven and hell.
The cult of Aten prospered in Egypt during the 12th Dynasty around 1900 BCE. They believed in the destruction of all other idols whom they recognized as valid deities but enemies of their preferred deity.
Hinduism developed around 2000 BCE in India and worshiped Brahman which they called ""One Being" or "One Truth". This led to a variety of different interpretations as some Hindus saw all their gods forming "One Being" or "One Truth". This is an example of very early monotheistic thought even though they are more polytheistic in the strictest form of the word.