Final answer:
The unity among ancient Greek city-states was largely due to shared religious beliefs and cultural practices, with Panhellenic sanctuaries and festivals like the Olympics playing a central role in promoting a united Greek identity.
Step-by-step explanation:
The unity among the city-states of ancient Greece was perpetuated primarily by shared religious beliefs. The ancient Greeks were connected by a common language, a shared pantheon of gods, and cultural practices, such as storytelling and symposia. Moreover, the Panhellenic sanctuaries and festivals, especially the Panhellenic games like the Olympics, were significant in uniting the Greek poleis culturally. These events were open to all Greeks, promoting interaction, competition, and a shared identity, despite their often fierce independence and frequent military conflicts.