Extract from The Penguin History of Greece by A.R.Burn, © 1969-85:

Orpheus first appears in Greek art and literature about 540 [BC]. . . . From the first he was associated with the . . . discontinuance of human sacrifice. . . . People with a sense of sin resorted to self-employed practitioners called 'Orphic initiators'. . . . Certainly they did not preach 'pay me and do as you like'; on the contrary, the severer Orphic doctrines taught (not always consistently as there was no Orphic church) respect for all life, with its corollary, vegetarianism, and sexual abstinence. The movement . . . remained . . . far into Christian times.

[Note: the author of the above book is not at all sympathetic to vegetarianism, though even he has to acknowledge it. However it is a useful introduction to the general history of the era - ed.]