The final phase of the Egyptian language and writing in which Greek script, with the addition of six new letters, was used to write ancient Egyptian. It is believed that the script was developed by Christians to spread their faith to the Egyptian populace. The script was used to translate Christian religious works, including the Bible, but also ordinary correspondence, business and legal texts, and funerary and other inscriptions in stone. Following the Arabic conquest in 642 AD, Coptic was gradually superseded by Arabic and fell out of use by the 16th century, with the exception of certain religious phrases no longer understood by the priests or general population. European scholars learned the language from exported Biblical and other religious manuscripts but were unaware that it was ancient Egyptian until the decipherment of the hieroglyphic script. Knowledge of Coptic greatly aided in the decoding and understanding of ancient Egyptian.
See also Champollion, Jean-Francois.
Historical Dictionary Of Ancient Egypt by Morris L. Bierbrier
Ancient Egypt. A Reference Guide. EdwART. 2011.