Kircher's most consequential work for the study of religion was Oedipus Aegyptiacus, a massive study published in four volumes between 1652 and 1654 that attempted a complete decipherment of Egyptian hieroglyphic writing. Kircher approached the hieroglyphs as a symbolic and mystical language encoding Hermetic and Neoplatonic wisdom, in keeping with the Renaissance belief, later associated with the Yates thesis, that Egypt was the fountainhead of an ancient theology transmitted through Hermes Trismegistus. On this basis he produced elaborate but, as later Egyptology conclusively showed after Jean-Francois Champollion's nineteenth century decipherment using the Rosetta Stone, systematically incorrect translations, since the hieroglyphs are in fact a largely phonetic writing system recording ordinary spoken Egyptian rather than a purely symbolic code.
Despite that central error, Oedipus Aegyptiacus remained the most thorough compilation of information on ancient Egyptian religion available to European scholars for nearly two centuries, and Kircher's underlying instinct that Egyptian religious thought deserved serious systematic study, rather than passing mention, anticipated the discipline of comparative religion that later scholars would build on firmer philological ground. His work is remembered today as a monument to erudition in service of a mistaken premise rather than as a reliable source on Egyptian belief itself.