First monotheistic culture

Answer 1

There are at least three contenders as the first monotheistic culture of the world.

Judaism

Some believe Judaism to be the first monotheistic religion. However, scholars now believe that Judaism, as a monotheistic religion, only became established in the middle of the first millennium BCE.

King Hezekia (729-686 BCE) of Judah made the first real attempt, in historic times, to impose a monotheistic religion, but his son soon allowed Judah to revert to polytheism. King Josiah ( c. 640-609 BCE) re-instituted religious reform and, in his reign, a monotheistic religion seems to have finally become dominant among the Jews. So, the earliest real evidence of a monotheistic religion in Israel and Judah is during the reign of King Hezekiah.

Egypt

Ancient Egyptians worship was a form of syncretistic polytheism, except briefly during the reign of Pharaoh Akhenaten , who became an uncompromising monotheist in the 14th century BCE. The economic distress that occurred at the end of his reign was seen as a warning from the gods and his successor restored the ancient beliefs.

Zoroastrianism

The Zoroastrian prophet, Zarathushtra (or Zoroaster) appears to have lived among the proto-Indo-Iranian people of southern Russia, somewhere around 1700 BCE.

Summary

On the evidence, Zoroastrianism was the first monotheistic religion in the world. The second was the brief Egyptian monotheistic religion that only lasted during the reign of Akhenaten. The third was in the Hebrew kingdom of Judah, during the reign of Hezekiah, but successfully re-instituted during the reign of Josiah.