What is the central belief of monotheism?

The central belief of monotheism is the belief in one God. Monotheism is characterized by the assertion that there is a single, supreme deity who is the creator and sustainer of the universe. This belief system stands in contrast to polytheism, which recognizes the existence of multiple gods, and atheism, which denies the existence of any gods altogether. Spiritualism, while it may involve the belief in a spiritual realm or beings, does not necessarily adhere to the idea of a singular divine entity. Therefore, the essential tenet of monotheism is the focus on the existence of one God, which underscores many of the world's major religions, including Judaism, Christianity, and Islam.