Monotheism can be defined as the belief in and worship of only one God. Although some early scholars of religion believed that monotheism may have been the earliest form of religion, it now seems certain that monotheism evolved gradually over a number of centuries. Certain monotheistic traits were displayed in both Zoroastrianism and, for a time, in ancient Egyptian religion. Generally, however, it is considered that monotheism originated in ancient Israel.
A form of religion that preceded monotheism in ancient Israel is henotheism. Henotheism can be defined as the recognition of many different gods, but the worship of only one God. In other words, one may acknowledge the existence of many gods, but worship one god exclusively. As late as the time of the prophet Micah (7th century BC), there are indications that the ancient Israelites thought largely in henotheistic terms. Micah says, "Every nation walks in the ways of its God; therefore, let us walk in the way of our God, O Israel." Clearly, the existence of other gods is not denied; rather Micah's argument is that those in the land of Israel should worship only the God of Israel.
Undoubtedly, there were many pivotal events in the emergence of monotheism. Judaism, Christianity and Islam consider Abraham to have played an important role in rejecting the worship of idols and many gods in favor of worship of only one God. Later, the law that was given to Moses set forth the notion that the God of Israel alone was to be worshipped: "You shall have no other gods before me." Moreover, in the revelation to Moses, there is the implicit notion that this God who demanded total allegiance was transcendent and beyond the control of humanity. When Moses asks the name of God the response is:
"I am who I am." This is only one translation of the Hebrew phrase. It may also mean I will be that which I will be" or "I will cause to be whatever I cause to be." Whatever the exact interpretation, the point is that this God of Israel is transcendent, not to be controlled by human whims and limitations. It is especially significant that God does not give a simple answer to the question of his name: in the ancient world, to know someone's name is to be able to exercise control (ritually or magically) over that being. Even the Hebrew phrase for the name of God (Yahweh) was not to be pronounced by the average person.
Notice that in both cases, there is not an explicit denial of the existence of other deities. It is clear, however, that both the Abraham and Moses traditions demand the exclusive worship of one, transcendent God.
The major event in the emergence of monotheism in Israel was the Exile. In 586 BCE, the Babylonians conquered Jerusalem and took most of its political and religious leadership into exile in Babylon. This loss of the temple, the priesthood and the land of Israel forced the Israelites to rethink the nature of God and the question of other gods. In particular, those in exile were forced to decide whether their God's power was limited to the boundaries of the land of Israel, or whether the power of their God extended far beyond those boundaries. What emerged by the end of the exile was the revolutionary notion that only one God exists. Moreover, this one God is not limited to one specific place, but was the creator and ruler of the entire universe.