Monism is not a religion or a faith per se. Instead, it is a metaphysical way of looking at the world. Monism holds that everything in the universe is really part of one substance or one nature. A monistic religion would hold that there is a god and that all of creation is really part of that god. Most scholars see Hinduism as an example of such a religion because Hindus believe that everything in the world is part of Brahman.
Polytheism can be monistic as well. In polytheism, there are held to be many gods. However, it is possible that these gods can all be seen as part of one overarching nature. Hinduism is also an example of such a religion. As this link shows, many Hindus believe in many gods, but see those gods' separate beings as somehow illusory since the gods are really part of one overall being.
But there are also purely polytheistic religions. A religion is polytheistic so long as it believes in many gods. It can believe that the many gods are all truly separate entities and therefore it can be polytheistic without being monistic.
Mono = one Theism = god
poly = many Theism = god
Christianity is an example of monotheism. God is soverign over everything and is in control over everything.
Eastern religions in India are an example of polytheism - they have different gods that do different things. They have one for fertility, etc.
Monism is a point of view within metaphysics which argues that the variety of existing things in the universe are reducible to one substances or reality and therefore that the fundamental character of the universe is unity. Monisms may be theologically syncretic by proposing that there is one god who has many manifestations in the diverse religious traditions.
Monism in philosophy can be defined according to three kinds:
a) Idealism: phenomenalism, or mentalistic monism which holds that only mind is real.
b)Neutral monism: which holds that both the mental and the physical can be reduced to some sort of third substance, or energy.
c)Physicalism or materialism: which holds that only the physical is real, and that the mental or spiritual can be reduced to physical.
Polytheism is the belief of multiple deities also usually assembled into a pantheon of gods and goddesses, along with their own methologist and rituals.Polytheism is the belief that there are many gods. Breaking the word down, “poly” comes from the Greek word for “many,” and “theism” from the Greek word for “God.” Polytheism has perhaps been the dominant theistic view in human history. The best-known example of polytheism in ancient times is Greek/Roman mythology (Zeus, Apollo, Aphrodite, Poseidon, etc.). The clearest modern example of polytheism is Hinduism, which has over 300 million gods. Although Hinduism is, in essence, pantheistic, it does hold to beliefs in many gods. It is interesting to note that even in polytheistic religions, one god usually reigns supreme over the other gods, e.g., Zeus in Greek/Roman mythology and Brahman in Hinduism.