Complex sentence: The Greek alphabet, which had been popularized throughout the eastern Mediterranean region during the Hellenistic period, remained the primary script of the Eastern Empire through the Byzantine Empire until its demise in the 15th century.
Split into simpler sentences: The Greek alphabet, which had been popularized throughout the eastern Mediterranean region during the Hellenistic period, remained the primary script of the Eastern Empire. Greek language and writing continued to be actively used in the Byzantine Empire until its demise in the 15th century.