Some insights on the background reading:
1) The Greeks recognized especially that music had an influence over the soul. In the cults of Cybele and Dionysus, music is used to stir the soul into a frenzy, so that you can be ecstatic (ek + stasis = to be beside yourself), which is a proper state to be able to receive the god and to be in touch with your inner self. Orpheus used music too, but his music entrances and calms. In the underworld the tortures stop, trees uproot themselves to follow him; his music purifies the soul of its evil, harmful elements.
2) Like many, if not all, of the major heroes of the ancient Greeks, Orpheus descended into the underworld and returned. This is always a symbol that the hero conquered death, something the ordinary human could never do. He went down there to retrieve Eurydice, and Hades and Persephone were so charmed by his music that they agreed to let him take her. But there was one condition: He couldn't turn around and look at her on his way out. (like Lot's wife, who turned into a pillar of salt when she looked at the destruction of Sodom and Gomorrah) Significance: He saw the process of regeneration, and thus knew too much. It is for this reason, primarily, that Orpheus became a fitting figure to build a cult around. We don't know who did it, but by around 600 B.C. the name of Orpheus is at the center of a cult, called Orphism.
There has come down to us a literature that has been ascribed to Orpheus himself, Orpheus the Theologian (but, of course, it's not really his): hymns, poetry, commentaries. Central to the Orphic religion was Dionysus, usually under the name Zagreus, and the stories they told about the birth of the gods and the creation of humans. Their theogony runs like this:
The first principle was Chronus (Time), sometimes described as a monstrous serpent having the heads of a bull and a lion with a god's face between; Chronus was accompanied by a brooding Adrasteia (Necessity), and from Chronus came Aether, Chaos, and Erebus. In Aether Chronus fashioned an egg that split in two; and from this appeared the firstborn of all the gods, Phanes, the creator of everything, called by many names, among them Eros.
He was a bisexual deity, with gleaming golden wings and four eyes, described as possessing the appearance of various animals. Phanes bore a daughter, Night, who became his partner in creation and eventually his successor in power. Night then bore Gaea and Uranus, and they produced the Titans. Next Cronus succeeded to the rule of Night and subsequently (as in Hesiod) Zeus wrested power from his father Cronus.
Then Zeus swallowed Phanes, and with him all previous creation (including a special race of human beings of a golden age); Zeus now created everything anew, with the help of Night. As second creator, Zeus became the beginning and middle and end of all things. Eventually Zeus mated with Kore and Dionysus was born. Hera was jealous of the child, and so she convinced the Titans to attack Dionysus and tear him apart and eat him. Zeus struck them down with a thunderbolt, but saved the heart of Dionysus. Out of the ashes human beings were born, born out of the evil of the Titans. But Zeus gave the heart of Dionysus to Semele, who swallowed the heart, and Dionysus was born again (twice-born). From then on, humans had their original evil side, their original sin, and a good side, coming from Dionysus, whose heart Semele had swallowed. Dionysus, the Orphics believed, could intercede on human's behalf with his mom, Persephone, who was angry at the children of the Titans.
For the Orphics, the goal was to try to purify oneself of one's Titan, monstrous self in favor of one's Dionysiac self. This ultimately required several lifetimes and steps of purity, reincarnation, special incantations and charms, before you were ready to escape the cycle and live in the Isles of the Blessed with the other pure souls.
The Orphics taught a life of ascetism (self-denial, which eventually led them to be ridiculed), including vegetarianism (because they emphasized the sanctity of life, the need to keep yourself free of blood and murder, and because of the transmigration of souls). They dressed in white, practiced frequent purifications and expiations, professed a creed.