Although the ancient Egyptians did not worship the Nile River, it was extremely important to their religious beliefs. Modern scholars believe that the Nile figured so prominently in Egyptian religion because it was so important to their lives on Earth.
Historians believe that the way that the Nile behaved did much to shape the religious beliefs of the Egyptians. The Nile was very predictable. Every year, it would flood at about the same time. The floods were not typically destructive. Instead, they actually brought life to the land. Historians argue that this influenced the Egyptians and led them to believe that the cosmic world was orderly and relatively benign as well. The Egyptians believed in the idea of Ma’at, a fixed, eternal order for the universe. They believed that Ma’at was under attack and needed to be sustained by their offerings and rituals. In other words, they had a world view that emphasized order and routine, just as the Nile acted in orderly ways. Their religion was meant mainly as a way to keep that orderly system working.
The Nile influenced religion indirectly, in that it shaped the conditions in which ancient Egyptians developed their religious ideas. For example, people living along the Nile encountered hippopotamuses, and one of their goddesses (Taweret) took hippopotamus form. More broadly, some have argued that the regularity of the Nile's flooding contributed to the religious idea of Ma'at, the regular order of things.
But the Nile -- essential for agriculture, and therefore for life itself -- also played an explicit role in certain aspects of Egyptian religious beliefs. Here are some examples.
1. Associations with Osiris. People relied on the floods to grow crops, and this gift of the Nile was linked with Osiris. According to ancient Egyptian mythology, the god Osiris was drowned in the Nile. The annual inundation was portrayed as life-giving water emanating from from Osiris's body. This point is discussed at length by Terje Oestigaard:
"The Nile water was supposed to have special life-giving virtues (Aldred 1984: 59). The vitality emerging from earth, either in plant or the water of the Nile, was seen as a manifestation of Osiris. Moreover, the different types of waters had specific qualities, and in particular the inundation: ‘The water of inundation which carried the silt was called the “pure water” or the “young water”, and it is this water that was thought to be brought by Osiris or to emanate from him or to take its power from him’ (Frankfort 1948: 190). This embodiment of the Nile was believed to be real and intimately connected to death, and in fact, the waters were the divine blood and life-juices from which everything and all life arose. The inundation was seen as the liquids running from Osiris’ decaying corpse."
2. Divine power of the kings. The ancient Egyptian pharaohs were worshiped as gods, and one of their divine powers was the ability to control the annual inundation of the Nile. Original texts portray pharaohs using their divine influence to maintain Ma'at, the cosmic order, and ensure that the Nile flooded each year. In some cases, there are original records stating that the god-kings were the direct cause of the inundations. For instance, in the pyramid texts of the Pharaoh Unas, it says:
"Unas is he who has caused the land to be under water [Nile flood], after he came out of the Lake…"
3. Hapi, god of the flooding of the Nile. The ancient Egyptian pantheon included Hapi, god of the annual inundation of the Nile. He was often depicted with a large belly and pendulous breasts, symbols of the fertile soil and nourishment that the flooding gives rise to.
In addition to the links below, see:
Oestigaard, T. 2010. Osiris and the Egyptian Civilization of Inundation; The Pyramids, the Pharaohs and their Water World. In Tvedt, T, & Coopey, R. (eds.). A History of Water, Series 2, Vol. 2: From Early Civilizations to Moderen Times: 72-99. I.B. Tauris. London.
Pyramid texts of Unas.