What was the religion was the sumerians

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:
Anu 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 in Sumerian myth was thought to have given humans 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).[28]
Ziggurats (Sumerian temples) consisted of a forecourt, with a central pond for purification.[citation needed] 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.[citation needed]

From Wikipedia