The Copts are a community in Egypt whose church goes back a very long way. Coptic is the adjective that points at them and at whatever is theirs. So a Coptic church is their church building, Coptic art is the art made in that tradition, the Coptic calendar is the one they count their year by, and Coptic writing is their old language written down. The word always starts with a capital C, because it is built on the name of a people. It does not mean Egyptian in general.

Coptic is the last stage of the Egyptian language, and it is written with letters borrowed from Greek. It stopped being an everyday language long ago, so it is called extinct or dead. It has not disappeared, though. Priests and choirs still read and sing it in church, and scholars still study it, which is why the word turns up in history books and in museum labels next to old papers. As a language name it takes no the and no plural: you study Coptic.