Beliefs on the afterlife evolved over time, with each major notion first articulated for the king and reserved for his use alone. These ideas were subsequently available to non-royal persons. Successive and divergent theories oft overlapped for centuries without being merged or fully rationalized.
The top-to-bottom spread of access to the next world in Egyptian society is called the democratization of the afterlife. The journey to immortality was dangerous and difficult and thereby required the assistance and support of the Gods.
The assistance and support of the Gods was at first attainable only by the Pharaoh. During the Old Kingdom, pyramid texts were inscribed upon the walls of pharaohic tombs and served as a passport. In the later part of the Old Kingdom, however, politicians and relatives near to the pharaoh began to piggyback, claiming that they were necessary for the pharaoh to lead a fulfilling afterlife. By building tombs near the pharaoh's pyramid they would gain immortality by association.
The Old Kingdom became weak due to civil war and a new religious doctrine (where the masses worshipped the pharaoh, who was the one and only able to worship the Gods directly), but what truly caused it to crumble was a cold spell which prevented precipitation and resulted in a lack of Nile floods. Famine ensued for decades, and from this point on Egypt was plagued by civil unrest. This resulted in a change in public perception of the pharaoh. While once a divine entity capable of enacting any decision (the pharaoh's word was law), the pharaoh was now considered a king imbued with the responsibilities of upholding justice and order. After so many years of havoc and repression, the commoners needed to feel more equality and order. At this point, anybody able to afford a tomb and mummificaiton could attain immortality with the god Osiris. Association with Osiris was accomplished by worshipping him at his temple, living an ethical life and participating in the rituals and festivals of the Osiris cult. Coffin texts were used as afterlife passports for the masses, which were based on Osiris myth rather than the pharaoh's formulae based on sun-god mythology). These coffin texts were placed directly on the mummy cases.
Capitalist motivations eventually meant that anybody could attain immortality. Scribes selling cheap and shabby copies of the Book of the Dead (a new reincarnation of the pyramid texts) allowed even the poorest persons to enter the afterlife. This was a very profitable market and indulged the renewed love of life which had been suppressed by the desperate years of famine just decades prior.
The Pyramid Texts of the late Old Kingdom are the earliest written sources regarding Egyptians' beliefs on the afterlife. Their beliefs undoubtedly arose before this, though written information is not available any earlier. The dead king was identified with Osiris, and thus believed to experience rebirth just as the murdered god had done.
The king's path to new life underwent democratization and was available to all Egyptians. Each Egyptian could now be individually identified with Osiris. From this time onward, the names of the dead were preceded by Osiris. For example, the The Osiris N became synonymous with The Deceased N. This association meant that the deceased could experience resurrection. The burial ritual and equipment promoted this transformation, with the deceased even referred to as Osiris in texts.