I've written previously about a policy of Arabisation in Egypt that gradually pressed out Greek & Coptic. I'm pulling the below from my reply in this earlier thread talking about the history and interactions between Coptic, Greek, and Arabic in Egypt.

Language

Throughout the 5th century through the 10th, there were three main languages in Egypt: Greek, Coptic, and Arabic. Gradually, Arabic came to replace both Greek and Coptic as both official languages of administration and instruction as well as the mother-tongue of the population at large.

Greek had been the language of administration prior to the conquests and remained so for years afterward. Even when there were formal attempts by the Caliphs to curtail the use of Greek, such as happened in 705, it often took years to be implemented as Greek was so ingrained into the administrative system. The first all-Arabic protocol is only dated to 732, 27 years after the decree, and the last bilingual protocol to 734. Greek use within the Coptic church continued until around the 8th century when the elites started to instead learn Arabic.

Interestingly, the use of Coptic increased in the years following the Arab invasions, at least within rural administration. It also increased within the Church itself, with aspects of religious services being performed in Coptic (a fact that continues until this day).

So what led to the eventual replacement of Greek and Coptic by Arabic? One factor was the increase in conversions from Christianity to Islam. Converts faced societal and religious pressures to learn Arabic. They also tended to move away from their rural homes and into cities, where they interacted in Arabic more so than in Coptic, which became unnecessary to know as a Muslim in Egypt. In addition to these societal pressures, the aforementioned decree of 705 meant that even Coptic elites began to instruct their children in Arabic, in order to ensure that they could secure administration posts.

Coptic then declined within the Church itself in the mid-tenth century, as Patriarch Christodoulos sanctioned Arabic translations of Coptic liturgies. As time progressed, Coptic became a language preserved in certain Church writings but used less and less frequently in actual social situations, leading to its decline. Eventually, even Coptic clerics began to write in Arabic as opposed to Coptic.

In sum, the religious changes within Egypt were not the result of a top-down policy by the Caliphs. People were not “forced” to convert so much as there were incentives, both economically and socially, to do so. These conversions then helped facilitate the rise of Arabic as a language over that of Greek or Coptic. Coupled with the decree of 705 (one of the few examples of forced change, although restricted to the bureaucracy), Arabic gradually became both the language of the elites and that of the common man.

There is a very thorough book on this subject, From Byzantine to Islamic Egypt: Religion, Identity and Politics after the Arab Conquest by Maged S. A. Mikhail, that I sourced most of this from. Unfortunately, it is a bit expensive if you don't have access to it from a university/other program.