The longest of the Homeric Hymns tells the abduction of Persephone by Hades, Demeter's search, her withdrawal from Olympus, and the compromise by which her daughter spends part of each year below. Read as myth it explains the seasons. Read as what it actually is, it is something more specific: an account of the founding of the Eleusinian Mysteries.
The hymn brings Demeter, disguised as an old woman, to Eleusis, has her received by the daughters of Keleos, sit in grief, refuse wine and ask for a mixed drink of barley and pennyroyal, nurse the king's son and attempt to make him immortal in the fire, be interrupted, reveal herself, and demand a temple. At the end she teaches the rites to the Eleusinian nobles. The kykeon, the fasting, the sitting on a stool, the ritual joking of Iambe: these are elements of the initiation, embedded in narrative.
That makes the hymn the single most important literary source for a cult that ran for something like a thousand years and about which the participants were forbidden to speak. Initiates included Athenians of every class, and slaves and women were admitted. Disclosure was a capital matter, Alcibiades was prosecuted for profaning the Mysteries.
What the hymn does not do is describe the culminating vision. It says that whoever has seen these things is blessed, and that the uninitiated has no share in such things after death, and it stops. The secret held.
The composition is usually placed in the seventh century BCE. The ascription to Homer is ancient and wrong.