As in the rest of Africa, the people of ancient Egypt were polytheistic throughout the Old Kingdom, the Middle Kingdom, and the New Kingdom. That means that they believed in many gods. Some of these gods were Ra, Anubis, Seth, Osiris, Isis, and Horus. Egyptians worshipped these gods with animal sacrifices and with incense and many processions where people carried the image of the god from one place to another.
People believed that all of Egypt belonged to the gods, and that the Pharaoh was the representative on earth of the gods, or maybe a kind of god himself, and so everything in Egypt sort of belonged to the Pharaoh. They thought that when you died, Anubis would weigh your soul against a feather, and if your soul was heavier than the feather (with bad deeds), you would be punished. They thought that after you died you went to a new world, just like this one, and so they put into your grave everything you would need in the next world.
But, as in Mesopotamia, there was also a little monotheism in Egypt. During the New Kingdom, the Pharaoh Akhenaten started a new worship of the god Aten, and he seems to have wanted people to believe that Aten was the only real god, or maybe the only god worth worshipping. After Akhenaten died, people went back to worshipping Anubis, Isis, Amon, and Osiris again, as they had before.
The Persian invasion of Egypt in 539 BC doesn't seem to have made any difference to Egyptian religion. The Egyptians just kept right on worshipping their own gods. But the Persians prided themselves on their religious tolerance. When Ptolemy took over Egypt in 323 BC, that did make a difference. Under Greek rule, the Egyptians did begin to worship some Greek gods, although they kept on worshipping the old Egyptian gods as well. Also at this time, Greek people in Athens began to worship the Egyptian goddess Isis. They learned about Isis from traders sailing over from Egypt.
When the Romans
conquered Egypt in 30 BC, again the Egyptians kept on worshipping their
own gods while at the same time continuing to worship the Greek
gods, and adding on some Roman
gods as well. If someone is powerful enough to conquer you, after
all, it might seem smart to worship their gods!
But little by little some Egyptians began to convert to Christianity, and by the time of the Great Persecution in 303 AD, there were many Christians in Egypt. After the Roman Emperors became Christian and the persecution ended, most of the people of Egypt seem to have converted to Christianity. This is the time of the great conflict between Arius and Athanasius, a good deal of which took place in Alexandria, in Egypt.
Around this time, the idea of the hermit started in Egypt, where holy men and women would leave their families, their jobs, their farms, and travel out into the desert beyond the Nile, and stay there devoting themselves entirely to Christ. When there got to be a lot of these hermits, they started banding together, and these are the first monasteries (the first monasteries had both men and women).
But with the coming of Islam to Egypt in the late 600s AD, most Egyptians soon converted from Christianity to Islam. Some Jews living in Egypt remained Jewish, and some of the Christians remained Christian - these Egyptian Christians are called the Copts, today. But since 700 AD most people in Egypt have followed the Islamic faith.
Gods and Goddesses of Ancient Egypt, by Leonard Fisher (1999). For younger kids.
Make This Egyptian Mummy, by Iain Ashman (2002). A project for kids.
The Egypt Game (Yearling Newbery), by Zilpha Keatley Snyder (reprinted 1985). A great kids' story about kids who pretend to be Egyptian gods and goddesses.
Religion in Ancient Egypt: Gods, Myths, and Personal Practice, by John Baines, David Silverman, and Leonard Lesko (1991). Pretty hard going, but it will tell you everything you need to know about Egyptian religion.
Akhenaten: Egypt's False Prophet, by Nicholas Reeves (2001). Reeves used to be a curator at the British Museum.
Coptic Egypt : Christians of the Nile, by Christian Cannuyer (2001). Clear, easy, with lots of pictures.