Monotheism is the belief in a single, universal, all-encompassing deity. Various forms of monotheism exist, including:
Most monotheists would say that, by definition, monotheism is incompatible with polytheism. However, devotees within polytheistic religious traditions often behave like monotheists. This is because a belief in multiple gods does not imply the worship of multiple gods. Historically, many polytheists believe in the existence of many gods, but worship only one. This practice is termed Henotheism. There are also monotheistic theologies within polytheistic cultures, such as some schools of thought within Hinduism which teach that the many gods merely represent aspects of a single or underlying divine power. Worship of a single god within a pantheon may also evolve into a form of monotheism, as in the case of the Aten cult in the reign of the Egyptian pharaoh Akhenaton.
Zoroastrianism is considered by some to be the earliest monotheistic view to have evolved among mankind, though it is not fully so, as the chief god Ahura Mazda is not the sole creator. It has been theorized that Judaism was influenced by Zoroastrianism as well as by Greek philosophy before arriving at its modern monotheistic view of God. Earlier Judaism is assumed to have claimed only that Yahweh was a tribal deity who took care of the descendants of Abraham. This view is not compatible with the self-understanding of the Abrahamic religions - Judaism, Christianity, Islam - which traditionally insist that exclusive monotheism is the original religion of all mankind, all other gods being viewed as idols and creatures which wrongly came to be worshipped as deities.
The Christian belief in the Trinity is monotheism, the worship of the one God of Abraham according to Trinitarian tradition. However, many Jews, Muslims, and unitarian Christians question this classification. Such critics claim that the Trinity is in fact a form of Tritheism, a hypothetical belief system which teaches that there are three gods -- that is, that the Father, the Son, and the Holy Ghost are, specifically, three distinct Gods.