Those that are still in use today tend to show highly diglossic characteristics in areas where they are used, as the difference between spoken and written language has widened over time.
Greek was the language of Homer and of classical Athenian, Hellenistic and Byzantine historians, playwrights, and philosophers.
It has contributed many words to the vocabulary of English and many other European languages, and has been a standard subject of study in Western educational institutions since the Renaissance.
In Western and Central Europe and in parts of northern Africa, Latin retained its elevated status as the main vehicle of communication for the learned classes throughout the Middle Ages and subsequently; witness especially the Renaissance and Baroque periods.
In terms of worldwide cultural importance, Edward Sapir in his 1921 book Language extends the list to include classical Chinese, Arabic, and Sanskrit: When we realize that an educated Japanese can hardly frame a single literary sentence without the use of Chinese resources, that to this day Siamese and Burmese and Cambodgian bear the unmistakable imprint of the Sanskrit and Pali that came in with Hindu Buddhism centuries ago, or that whether we argue for or against the teaching of Latin and Greek [in schools,] our argument is sure to be studded with words that have come to us from Rome and Athens, we get some indication of what early Chinese culture and Buddhism, and classical Mediterranean civilization have meant in the world's history.