Monotheism is the belief in only one god, rather than two or more, which would be polytheism such as Ancient Greece, Ancient Rome, and Ancient Egypt. Many religions are monotheistic, such as Judaism, Christianity, Islam, Baha'i Faith, Sikhism, Eckankar, Zoroastrianism, deism, Cao Dai and Tenrikyo. Zoroastrianism is among the oldest monotheistic religions.
Monotheism for Kids. Kiddle Encyclopedia.