Sumerian religion has its roots in the worship of nature, such as the wind and water. The ancient sages of Sumer found it necessary to bring order to that which they did not understand and to this end they came to the natural conclusion that a greater force was at work. The forces of nature were originally worshipped as themselves. However, over time the human form became associated with those forces. Gods in human form were now seen to have control over nature.
The Sumerians regarded the universe as consisting of heaven and earth. The Sumerian term for universe is an-ki, which translates to "heaven-earth." Earth was seen as a flat disk surrounded by a hollow space. This was enclosed by a solid surface which they believed was made of tin. Between earth and heavens was a substance known as lil, which means "air" or "breath." The moon, sun, stars, and planets were also made of lil, but they were also luminescent. Completely surrounding the an-ki was the primeval sea. The sea gave birth to the an-ki, which eventually gave rise to life.
Sumerian theologians believed that every intricacy of the cosmos was controlled by a divine and immortal being. The cosmos adhered to established rules.
The world below was known as the nether world. The Sumerians believed that the dead descended into the nether world, also known as the under world. The souls of the dead entered the nether world from their graves, but there were also special entrances in cities. A person could enter the nether world from one of these special entrances, but could not leave unless a substitute was found to take their place in the world below. A person entering the nether world must adhere to certain rules:
Failure to adhere to these rules would cause the person to be held fast by the denizens of the nether world until a god intervened on their behalf.
The nether world was ruled by Nergal and Ereshkigal. They had at their disposal a number of deities, including a number of sky-gods who feel out of faith with later Sumerian theologians.
After descending into the nether world a soul had to cross a river with the aid of a boatman who ferried them across. They then confronted Utu, who judged their soul. If the judgment was positive the soul would live a life of happiness. It was, however, generally believed by Sumerians that life in the nether world was dismal.
The gods of Sumer were human in form and maintained human traits. They ate, drank, married, and fought amongst each other. Even though the gods were immortal and all-powerful, it was apparent that they could be hurt and even killed.
Each god adhered to a set of rules of divine authority known as me. The me ensured that each god was able to keep the cosmos functioning according to the plans handed down to them by Enlil.
Hundreds of deities were recognized in the Sumerian pantheon. Many were wives, children, and servants of the more powerful deities. The gods were organized into a caste system. At the head of the system was the king or supreme ruler. The four most important deities were An, Enlil, Enki, and Ninhursag. These were the four creator deities who created all of the other gods. An was initially the head of the pantheon, though he was eventually seceded by Enlil. Enlil is seen as the most important god. He is known as "the king of heaven and earth," "the father of the gods," and "the king of all the gods." Enlil developed the broad designs for the universe. However, it was Enki who further developed and carried out his plans. Ninhursag was regarded as the mother of all living beings.
Under the four creator deities were the seven gods who "decree the fates." These were An, Enlil, Enki, Ninhursag, Nanna, Utu, and Inanna. These were followed by the 50 "great gods" or Annunaki, the children of An.
Here is a compilation of the Sumerian deities.
Sumerians believed that their role in the universe was to serve the gods. To this end the ancient Sumerians devoted much of their time to ensuring their favor with the gods with worship, prayer, and sacrifice. The high gods, however, were believed to have more important things to do than to attend to the common man's every day prayers, and so personal gods were devised as intermediaries between man and the high gods. The personal gods listened to the prayers and relayed them to the high gods.
The temple was the center of worship. Each city usually had a large temple dedicated to their patron god, and might also have small shrines dedicated to other gods. Daily sacrifices were made consisting of animals and foods, such as wine, beer, milk, and meats. Additionally special occasions called for spectacular festivities that would sometimes last for days. Special feasts took place on the day of the new moon, on the 7th, 15th, and last day of the month. However, the most important day by far was the New Year.
The head of the temple was called the sanga. The sanga was responsible for ensuring the temple's finances, buildings, and day-to-day activities were all in good order. The en was the spiritual leader of the temple. The en could be a man or woman depending upon the deity. Under the en were various priest classes, such as the guda, mah, gala, nindingir, and ishib. The roles of all of these classes is not known, though the ishib was in charge of libations, and the gala was a poet or singer.
The city's main temple was usually dedicated to their patron deity. Patron deities often assumed the powers of other deities, which tended to result in confusion and contradiction in the literature of ancient Sumer. For example, ancient legends would often change to reflect the new-found popularity of a particular god. If Marduk rose to prominence, then certain legends would alter to reflect such.