This account of the origin of the Eleusinian Mysteries is not indeed capable of demonstration, but it seems to me at least the most probable in itself, and the most conformable to the habits of the Greeks, as to those of all early nations.
Certain it is that for a long time the celebration of the Eleusinian ceremonies was confined to these two neighbouring states, until, as various causes contributed to unite the whole of Greece in a common religion and a common name, admission was granted all Greeks of all ranks, male and female,--provided they had committed no inexpiable offence, performed the previous ceremonies required, and were introduced by an Athenian citizen.
With the growing flame and splendour of Athens, this institution rose into celebrity and magnificence, until it appears to have become the most impressive spectacle of the heathen world.

It is evident that a people so imitative would reject no innovations or additions that could increase the interest or the solemnity of exhibition; and still less such as might come (through whatsoever channel) from that antique and imposing Egypt, which excited so much of their veneration and wonder.

Nor do I think it possible to account for the great similarity attested by Herodotus and others, between the mysteries of Isis and those of Ceres, as well as for the resemblance in less celebrated ceremonies between the rites of Egypt and of Greece, without granting at once, that mediately, or even immediately, the superstitious of the former exercised great influence upon, and imparted many features to, those of the latter.

But the age in which this religious communication principally commenced has been a matter of graver dispute than the question merits.

A few solitary and scattered travellers and strangers may probably have given rise to it at a very remote period; but, upon the whole, it appears to me that, with certain modifications, we must agree with Lobeck, and the more rational schools of inquiry, that it was principally in the interval between the Homeric age and the Persian war that mysticism passed into religion--that superstition assumed the attributes of a science--and that lustrations, auguries, orgies, obtained method and system from the exuberant genius of poetical fanaticism.
That in these august mysteries, doctrines contrary to the popular religion were propounded, is a theory that has, I think, been thoroughly overturned.