It is not surprising that the religious beliefs of the Sumerians changed during the long period of their history. According to Sayce:

"In historical Babylonia the gods were conceived of in the form of man. Man was created in the image of God because the gods themselves were men. But the conception cannot be traced back further than the age when the Sumerians and Semites came into contact with one another. In pre-Semitic Sumer there are no anthropomorphic gods. We hear, instead, of the zi or 'spirit', a word properly signifying 'life' which manifested itself in the power of motion. All things that moved were possessed of life, and there was accordingly a 'life' or 'spirit' of the water as well as of man or beast. .... Sumerian theology, in fact, was still on the level of animism... Vestiges of the old animism can still be detected even in the later cult : by the side of the human gods an Assyrian prayer invokes the mountains, the rivers and the winds, and from time to time we come across a worship of deified towns. It was the town itself that was divine, not the deity to whom its chief temple was dedicated. So, again, the god or goddess continued to be symbolized by some sacred animal or object whose figure appears upon seals and boundary-stones..."

"With the advent of the Semite all is changed. The gods have become men and women with intensified powers and the gift of immortality, but in all other respects they live and act like the men and women of this nether world. ... The Semitic god of Babylon was 'lord of gods' and men, of heaven and earth; Assur of Assyria was 'king of the gods' and lord of 'the heavenly hosts'."

"It was natural that, corresponding with this lord of the heavenly hosts, there should be a lord of the hosts of earth, and that as the divine king was clothed in the attributes of man, the human king should take upon him the divine nature. Like the Pharaohs of Egypt or the emperors of Rome, the early kings of Semitic Babylonia were deified. And the deification took place during their life-time, in fact, so far as we can judge, upon their accession to the throne. In the eyes of their subjects they were incarnate deities, and in their inscriptions they give themselves the title of god."

There was no organized set of gods; each city-state had its own patrons, temples, and priest-kings. The Sumerians were probably the first to write down their beliefs, which were the inspiration for much of later Mesopotamian mythology, religion, and astrology.

The Sumerians worshipped:

• An as the full time god, equivalent to "heaven" - indeed, the word "an" in Sumerian means "sky" and his consort Ki, means "Earth".
• Enki in the south at the temple in Eridu. Enki was the god of beneficence, ruler of the freshwater depths beneath the earth, a healer and friend to humanity who was thought to have given us the arts and sciences, the industries and manners of civilization; the first law-book was considered his creation,
• Enlil, lord of the ghost-land, in the north at the temple of Nippur. His gifts to mankind were said to be the spells and incantations that the spirits of good or evil were compelled to obey,
• Inanna, the deification of Venus, the morning (eastern) and evening (western) star, at the temple (shared with An) at Uruk.
• The sun-god Utu at Sippar,
• the moon god Nanna at Ur.

These deities were probably the original matrix; there were hundreds of minor deities. The Sumerian gods thus had associations with different cities, and their religious importance often waxed and waned with those cities' political power. The gods were said to have created human beings from clay for the purpose of serving them. If the temples/gods ruled each city it was for their mutual survival and benefit—the temples organized the mass labor projects needed for irrigation agriculture. Citizens had a labor duty to the temple which they were allowed to avoid by a payment of silver only towards the end of the third millennium. The temple-centered farming communities of Sumer had a social stability that enabled them to survive for four millennia.

Sumerians believed that the universe consisted of a flat disk enclosed by a tin dome. The Sumerian afterlife involved a descent into a gloomy netherworld to spend eternity in a wretched existence as a Gidim (ghost).

Ziggurats (Sumerian temples) consisted of a forecourt, with a central pond for purification.The temple itself had a central nave with aisles along either side. Flanking the aisles would be rooms for the priests. At one end would stand the podium and a mudbrick table for animal and vegetable sacrifices. Granaries and storehouses were usually located near the temples. After a time the Sumerians began to place the temples on top of multi-layered square constructions built as a series of rising terraces, giving rise to the later Ziggurat style.

There were many different types of priests. Some of the more common ones:

• āšipu an exorcist and physician
• bārû a diviner and astrologer
• qadištu a priestess and prostitute