The Passion of Osiris probably began in early times as a set of fertility rites. Later the emotional content took overhand and as such it remained, from the 3rd millennium B.C. throughout Ptolemaic texts from Philae or Denderah. To begin to understand why this drama was so important and so tightly knitted to the question of kingship, we must realize that for the ancient Egyptian the agricultural year was not as benign as in Western and European countries. The peoples there had a basic knowledge that even if there were bad years with failing of crops and famine, the new growth would always return next year so the whole of existence was not totally threatened.

For the ancient Egyptian it was different. The drought and heat of summer meant that the fertile strip of land by the Nile turned into desert, which for them equaled the place of death. The ordered world where life could prosper, turned every year into chaos and barrenness. Their world disappeared and they were never really sure that the flood would come back and in sufficient amount.

The Inundation or Sopdet

Sirius, or the Dog Star, in Egypt called Sopdet (Gr: Sothis), was believed to bring the inundation. In later times it was equaled to Isis. The inundation returned at the same time as Sopdet became visible above the horizon after 70 days of absence, it meant literally that death and destruction was staved off. And the inherent life force that was brought back by the inundation was Osiris. He is what sprouts when the water irrigate the fields but he can only do this with the help of Isis, in the aspect of Sopdet. And now there is time to rejoice!:

"I am the messenger of the year, for Osiris,
here I come with the news of your father, Geb,
The state of the year is good, how good it is!
The state of the year is fair, how fair it is!
I have come down with the Twin Companies of the gods
provider of the fields with plenty;
I have found the gods standing, clothed in their linen,
their white sandals upon their feet.
They throw off their sandals upon the ground,
they divest themselves of their fine linen;
`There was no happiness until you came down! they say.
`What is told you will abide with you!
Canal of happiness will be the name of this canal
as it floods the fields with plenty."

The reason for the popularity of the cult was probably that people could identify with the sufferings of Osiris and Isis. Apart from the emotional content of the story of the Passion (we will look at that later), here is a whole chain of reasons for celebrating Osiris. He was:

1: Fourth generation of gods as the great grandson of Atum,
2: King over Egypt in a past golden age, and teacher of cultivation, order and justice (since the Middle Kingdom),
3: Victim of his brother Set,
4: By the intervention of Isis, Father of the next King and Ruler of the Underworld,
5: Bringer of the powers of life by the means of his son Horus.

So the Passion of Osiris wraps it all up. All the mythic content is knit together to form the basis for the theocratic rulership of the land. In Egypt duality was always present and here we have Osiris and Horus functioning as two parts of a whole, in a complementary relationship: Without his son Horus, Osiris would not be redeemed, and without his father Osiris, Horus would not be able to assume rulership over Egypt. In this way the myth certifies and supports the Divine Rulership over Egypt. Most coronation ceremonies also brought out elements of the myth to confirm Pharaoh as the Living Horus and rightful King of Egypt.

And the link becomes clear - from the moment of Creation goes a straight line of descendancy to the ruling king, Son of Horus. The heavy task laid upon him by his heavenly ancestors was expressed in the daily duty of ascertaining the welfare of his people by honoring the gods and the principle of Ma´at by daily rituals and offerings. This was the basis for the continuance of existence, if the principle of Ma´at - justice, balance, order - was not upheld, the chaotic forces would overthrow existence on all levels of life.

Sources:
Myth and Symbol in Ancient Egypt by R.T. Rundle Clark, Thames & Hudson, pbk 1993
Egyptian Myths by George Hart, British Museum Press, 1997
Conceptions of God in Ancient Egypt by Erik Hornung, Cornell Paperbacks, 1996
Ancient Egyptian Literature by Miriam Lichtheim, University of California Press, 1976
Religion in Ancient Egypt, edited by Byron E. Shafer, Cornell University Press, 1991
Ancient Egyptian Religion by Stephen Quirke, British Museum Press, 1992