The Coptic alphabet is an alphabet used for writing the Coptic language. It is based on the Greek alphabet, but contains some extra letters for sounds used in Coptic but not in Greek. Those letters are derived from the Demotic script, an Egyptian alphabet.
The Coptic alphabet came into use during the 3rd century BC after the Greeks conquered Egypt. It is still used by the members of the Coptic Church to write their religious texts. All the Gnostic codices found in Nag Hammadi used the coptic alphabet.