Under Ottoman occupation the Church could no longer bear witness to Christ . Christian missionary work among Muslims was illegal and dangerous , whereas conversion to Islam was entirely legal and permissible . Converts to Islam who returned to Orthodoxy were put to death as apostates . No new churches could be built and even the ringing of church bells was prohibited . The Hagia Sophia and the Parthenon , which had been Christian churches for nearly a millennium , were converted into mosques . Education of the clergy and the Christian population either ceased altogether or was reduced to the most rudimentary elements . Violent persecutions of Christians were common , and reached their climax in the Armenian , Assyrian , and Greek genocides
Armenian
Armenian Genocide
History of Christianity