1821)
When the Ottomans arrived, two Greek migrations occurred. The first migration entailed the Greek intelligentsia migrating to Western Europe and influencing the advent of the Renaissance. The second migration entailed Greeks leaving the plains of the Greek peninsula and resettling in the mountains. The millet system contributed to the ethnic cohesion of Orthodox Greeks by segregating the various peoples within the Ottoman Empire based on religion.
The Greeks living in the plains during Ottoman domination were either Christians who dealt with the burdens of foreign rule or Crypto-Christians (Greek Muslims who were secret practitioners of the Greek Orthodox faith). Some Greeks became Crypto-Christians to avoid heavy taxes and at the same time express their identity by maintaining their ties to the Greek Orthodox Church. However, Greeks who converted to Islam and were not Crypto-Christians were deemed Turks in the eyes of Orthodox Greeks, even if they didn't adopt Turkish language.
The Ottomans ruled Greece until the early 19th century.
Read more about this topic: History Of Greece, Ottoman Rule (15th Century