Now, in the case of those Eleusinian mysteries, which are the very heresy of Athenian superstition, it is their secrecy that is their disgrace. Accordingly, they previously beset all access to their body with tormenting conditions; and they require a long initiation before they enroll (their members), even instruction during five years for their perfect disciples, in order that they may mold their opinions by this suspension of full knowledge, and apparently raise the dignity of their mysteries in proportion to the craving for them which they have previously created. Then follow the duty of silence.
Carefully is that guarded, which is so long in finding. All the divinity, however, lies in their secret recesses: There are revealed at last all the aspirations of the fully initiated, the entire mystery of the sealed tongue, the symbol of virility. But this allegorical representation, under the pretext of nature’s reverend name, obscures a real sacrilege by help of an arbitrary symbol and by empty images obviates the reproach of falsehood!
(Tertullian. Against the Valentinians I)
Why is the priestess of Demeter carried off, unless Demeter herself had suffered the same sort of thing? (Tertullian, To the Nations 30)
