Which document's decipherment depended on a Coptic liturgy still sung in churches?

Egyptian hieroglyphs. Coptic is the last stage of the Egyptian language, written in Greek letters and kept alive in the liturgy of the Egyptian church long after it stopped being spoken - so the sounds of the language survived, in a script that could be read, while the script that recorded it could not.

Champollion learned Coptic as a teenager and treated it as the living end of the language he was trying to read. The Rosetta Stone gave him the same decree in Greek and in two Egyptian scripts; Thomas Young had already shown that cartouches held royal names spelled phonetically. What Champollion added in 1822 was the recognition that the writing mixes sound signs and meaning signs rather than being one or the other, and that the sounds behind it were Coptic.

The obstacle had been a bad idea with excellent credentials. A late Greek author had described the signs as symbols of ideas, and Europe believed him for fourteen centuries, producing elaborate mystical translations of texts that actually say things like a list of temple offerings. Once a picture of a mouth could simply mean the sound r, three thousand years of records became legible inside a generation.