Coptic carries a heavy load of Greek vocabulary. Estimates vary by text and genre, but a Coptic biblical, theological or monastic work can draw a substantial minority of its words from Greek, some fully naturalised and some retaining Greek endings. That did not happen by accident or by pressure. The people who created Coptic Christian literature were translating from Greek scripture, Greek liturgy and Greek theology, and they took over the vocabulary of that material rather than inventing Egyptian equivalents for terms whose meanings were already settled by centuries of use. The result is a language in which Egyptian grammar carries Greek terminology, and letters like fai and phi mark where the seam runs.
The seam is visible elsewhere too. Coptic words for the ordinary business of life are Egyptian; words for doctrine, for the offices of the church, for kinds of virtue and vice, are often Greek. Documentary Coptic, the enormous body of letters, contracts, accounts and receipts surviving from monastic settlements, shows a lighter Greek load than literary Coptic does, which is what one would expect if the borrowing came in with the scriptures. Reading Coptic is therefore an exercise in reading two histories at once, and the alphabet, half Greek and half Egyptian, is doing the same thing.