Dialects
Now we see two independent attempts to write the Egyptian language in new script. Each attempt was unique in its motive, approach, and audience. Due to the distribution of the population along the length of the Nile, many dialects developed. Each was characterized by the use of different vowels in pronouncing the same words as well as some distinct variation in the vocabulary. The pagans attempted from the start to develop a uniform written language in a neutral Dialect, the Sahidic. Because of their early start, they were successful in their efforts and nearly erased any influence that such regional dialects had on their own version of Coptic. The Christians on the other hand put the benefit of the people ahead of proper language development and resurrected all these regional dialects in a written form. Eventually most of these dialects fell into disuse as the uniform Sahidic became the more dominant again.
All the dialects were to a large extent geographically-dependent. Their spanned the entire length of the Nile Valley. Based on literary records we have such dialects as the Akhmimic and the Lycopolitan (Asyutic) dialects of Upper Egypt, the Middle Egyptian and the Fayoumic of Middle Egypt, and the Bohairic of the Delta. Then there is the Sahidic dialect that became, from the earliest times, a neutral dialect used throughout Egypt and eventually gained literary dominance with the extensive writings of St. Shenouda the Archimandrite. There is also a host of minor dialects as well as subdialects to the ones mentioned above.
Today Bohairic is the only surviving dialect of Coptic. It was kept alive first by the strength of the monastic communities of Wadi n' Natrun (Scitis), which used it extensively. Then with the move of the Patriarchate from Alexandria to Cairo in the 11th century, Bohairic, the dialect of the District, became the official dialect of the Church replacing the Sahidic.

The Golden age of Coptic
Coptic was used from its Christian beginnings in the late second century A.D. till the time of the Great persecution of Diocletian in the early 4th century A.D. predominantly as a translational tool from Greek to Egyptian. After the persecution, the monastic movement picked up tremendous steam. It was for the Copts the only way they can express their great love for God, that they earlier expressed with the willing sacrifice of their most precious possession, their earthly lives. These monastic communities were large and mostly Egyptian. This generated the need for the abbots of these communities to write their rules in their Egyptian language. Furthermore, the Fathers of the Egyptian Church, who otherwise usually wrote in Greek, addressed some of their works to the Egyptian monks in Coptic.
Hence, with monastic fathers like St. Antony, St. Pachomius, and St. Macarius and their respective disciples writing to their monks; and with Church Fathers like St. Athanasius, St. Theophilius, and St. Cyril writing also to them in Coptic, the Golden Age of the Coptic language was about to begin.
It was not until St. Shenouda the Archimandrite came on the scene that Coptic really achieved its literary excellence. St. Shenouda who lived from 348 to 466 AD. was able to transform the language from a tool to communicate instructions to the monks to a wide-variety literary language that addressed monks, ecclesiastic authorities, laymen, and even government officials. His charisma, knowledge of Greek language and rhetoric, and his innovative mind gave him the necessary tools to elevate the Coptic language, in content and style, to a literary height never achieved before nor equaled since. The Coptic scholars are constantly astounded by his great writings as more and more of them are being studied and accurately published.
This literary legacy continued to a lesser degree through the writings of his disciple St. Besa in the second half of the fifth century. But such writings were mostly for the edification of the large monastic community in the White Monastery. Later in the sixth and seventh centuries other fathers wrote many works in Coptic like Rufus of Shotep, Constantine of Asyut, and Pisentius of Qift.

Coptic During the Early Arabic Period (7th to 10th Century A.D.)
By the middle of the seventh century, Egypt came under the dominance of Arab rulers that eventually tried to force the Copts to learn Arabic to keep their government jobs. This policy slowly eroded the number of Coptic lay readers who were mostly from the ranks of these government workers and their families. In other words the pressure put on such families to learn Arabic to ensure their continuing service in the government and the inheritance of such work by their offspring, made them slowly neglect educating their children in literary Coptic. Within a few hundred years Bishop Severus of Al-Ashmunain found it necessary to write his 'History of the Patriarchs' in Arabic to address such a drastic decline.
Ecclesiastically, the language continued strong. In fact, a great number of Hagiographic texts were composed during the early parts of this period. Coptic continued to be used in the Church with Greek as the second language, as seen from the texts that survived from the period. However a relatively small number of liturgical manuscripts survived from such period to show how it was being used. This was due to the heavy use that such manuscripts were subjected to, poor preservation during the period of decline in use, and the parchment material they were written on that did not lend itself to such heavy use.
During this period some Arabic loan-words made their way into the language. But there was no indication that the Arabic language was used in the Church. There were no Coptic-Arabic manuscripts that belong to this period or any literary citation to indicate its possible use. Coptic also remained the spoken language of the peasants and probably the clergy.