Egyptian religious material survives in enormous quantity and with a heavy bias: almost all of it is funerary, monumental or temple-based, which means it records what elites paid to have written on durable surfaces.
The oldest large body is the Pyramid Texts, carved inside royal pyramids at the end of the Old Kingdom, spells to secure the king's passage and standing after death. In the Middle Kingdom comparable material appears on the coffins of officials, and by the New Kingdom the tradition has become the collection of spells conventionally called the Book of Going Forth by Day, written on papyrus and available to anyone who could pay for a copy. Read in sequence these show a religion widening its access rather than changing its mind.
Temple inscriptions supply the other half. The very late temples at Edfu and Dendera carry long, systematic accounts of myth and ritual, and they are simultaneously the fullest and the latest evidence, written when Egypt was ruled from elsewhere and its priesthood was consolidating what it knew.
Two absences shape everything. There is little that records private belief as opposed to prescribed practice, though the workmen's village at Deir el-Medina is a partial exception. And the tradition ends within a documented span: the last dated hieroglyphic inscription is of the late fourth century of the common era, and the temple at Philae was closed in the sixth. After that the script was unreadable for over a thousand years, until the bilingual decree on the Rosetta Stone reopened it.