Before 1822 the religion of Egypt was known almost entirely at second hand, through Greek and Roman writers who described it from outside, and through a late work called the Hieroglyphica attributed to Horapollo, which explained each sign as a symbol of an idea. That book was the basis on which Renaissance scholars, Athanasius Kircher among them, built elaborate mystical readings of inscriptions they could not read at all, and behind those readings lay a much older European conviction that Egypt held a wisdom encoded in pictures. The conviction is itself an important object in the history of Western esotericism, and it was resting on a mistake about how writing works.
Afterwards the Egyptians could be heard. The Pyramid Texts, the coffin inscriptions, the collections of spells that a modern reader knows as the Book of the Dead, the temple rituals, the hymns to the sun, the letters written to dead relatives: all of it became legible in the language it was composed in. Almost everything the atlas records about Egyptian religion from Egyptian evidence rather than from Greek report dates from after this work, and the hermetic tradition that had been built on the symbolic assumption had to decide what to do about it.