The novel was probably written around 200 - 300 AD, but it is set in Achaemenid Egypt, when Egypt was a satrapy of the Persian empire (between 525 - 332 BC).
One of the things I love about this text is that the realities of language and ethnicity are everywhere. Greeks in Egypt must find another Greek (often a captured Greek slave) to act as translator. The wife of the Persian governor of Memphis, though she understands Greek, cannot speak it, so she too needs a translator to communicate with the Greeks. Phoenicians on trading expeditions are found in Delphi, and recognized at other islands in the Mediterranean by their huge and well-built ships.
One thread of the complex plot involves a mysterious letter written in an unknown language. This is in the possession of the main female character of the story, Chariclea, the adopted daughter of a Greek priest of Apollo. Among the belongings he received when he adopted her is a letter written on cloth that is neither in Greek nor Egyptian. This mysterious letter is finally read by a visiting Egyptian priest, who says:
"It was in Ethiopian letters, not the common sort, but such as their princes use, like to those that the Egyptians call hieratic."When Heliodorus tells us that they are "Ethiopian letters", however, he is not referring to the old Ge'ez abjad. Indeed, the Ge'ez letters do not look much like hieratic:
Ge'ez abjad, from Wikipedia
A piece of one of the Chester-Beatty papyri in hieratic
In fact, when Heliodorus says Ethiopia, he means the kingdom of Kush, and the writing system he is referring to must be the cursive form of Meroitic:
A Meroitic ostracon, from the Unesco website.
Meroitic is not yet fully deciphered, but the script is known, and it was in use for about 800 years from 300 BC - 600 AD. Its inclusion in the Aethiopica is probably a little anachronistic, but it is a sin that can easily be forgiven in such a well-written story from so long ago.