The Mysteries of Eleusis

The Eleusinian Mysteries was the cult of Demeter and Persephone which was based at Eleusis in Ancient Greece. The cult revolved around the myth of Persephone being returned to her mother from te underworld every spring. It was a major festival during the Hellenic Era for the Greeks (the festival even spread to Rome).

The Return of Persephone (1891)

Frederic Leighton

The rites, ceremonies, and rituals were kept a secret and the only persons allowed were:

1. Priests, priestesses, and Hierophants
2. Initiates, undergoing the ceremony for the first time
3. Those who have already been initiated, and were eligible for 
4. Those who had obtained ἐποπτεία (épopteia or "contemplation"), who had learned the secrets of the greatest mysteries of Demeter

Demeter

While the exact actions of the cult of Demeter and Persephone where not known to modern scholars and were not written down, from the art made of the Eleusinian Mysteries it is thought that they had three revelations during the initiation. The first of Persephone, then of Persephone giving birth in fire to a divine child named Aeon, and then of a ear of wheat, to represent the coming of a new year. 

Demeter (2012)

Jjlovely

These ceremonies were thought to hold the human races together, and were observed for 2,000 years and only came to an end with the sacking of Eleusis by the Goths in 396 BCE.

Votive plaque depicting elements of the Eleusinian Mysteries. National Archaeological Museum, Athens