The secret tradition may have been connected to Judeo-Christian apocalypses and the rites described in those texts, but Clement’s Letter to Theodore made numerous allusions to Greek mystery rites, the Eleusinian mysteries in particular. There were a number of Greek mystery cults that allowed individual to be initiated in the hopes of attaining a better afterlife, the most famous of which was at Eleusis a few miles from Athens. In the fall, Greeks could perform rites at Eleusis that, according to Cicero, taught people “how to live in joy, and how to die with better hopes.”
The allusions to the Eleusis in the Letter to Theodore are nicely explained by Scott Brown:
The Letter to Theodore’s imagery of entering the innermost sanctuary is a mixed metaphor, combining Greek mystery initiation language with Jewish mystical reflection on the veils and sanctuaries of the Jerusalem temple. The metaphor of a mystagogue leading initiates evokes the practice at Eleusis, where persons who had already undergone the great mysteries served as sponsors for the first-time initiates and led them into the Telesterion, the temple in which the spectacle of the great mysteries occurred. The letter’s reference to ‘the hierophantic teaching of the Lord’ further elaborates this metaphor by alluding to the hierophant who makes the sacred symbols appear during this spectacle. And ‘the things not to be uttered’ correspond to the secrets revealed in the Greek mysteries themselves, the contents of which initiates were prohibited from divulging. The mystic Gospel of Mark does not contain these things because they are too secret and sacred to be written down. The author, therefore, is intentionally employing imagery of the highest grade of a secret initiation.
Just to clarify a bit more: a mystagogue was a guide during the mysteries, the hierophant revealed the sacred objects of the mysteries, and the Eleusinian rites were divided between the small mysteries (purification rites prior to the mysteries) and the great mysteries (rites performed in the Telesterion, or temple). Clement used all these allusions not only in the Letter to Theodore but also throughout his writings.
Many other early Christian writers used the language of the mysteries, particularly St. Paul. The mysteries usually commemorated the death and redemption of a god or hero; Eleusis commemorated the myth of Persephone descent into the underworld and her assent and reuniting with Demeter. Those who were initiated at Eleusis underwent a ritual death, similar to how Paul described baptism.
“Some ancient Christian writers were struck by certain similarities between Christian worship and mysteries,” explains Walter Burkert, “and they denounced the latter as devilish counterfeits of the one true religion.” Clement himself denounced the mysteries in his Exhortation to the Greeks. In that work Clement went so far as to claim to reveal what the Eleusinian holy objects were, the ultimate sacrilege. Clement’s use of mystery terminology suggests that Clement viewed the mysteries in the way Burkert described: an imitation of Christianity. Furthermore, in Clement’s Letter to Theodore he called the rite when initiates were read Secret Mark, the “great mysteries.” Perhaps the rite had similarities to Eleusis or other mystery rites. Clement asserted continuously that the Greeks stole their true ideas from the Hebrews at some point or other and may have felt the same way about the mysteries.
 Walter Burkert, Ancient Mystery Cults (Cambridge: Harvard University Press, 1987), 21.
 Scott G. Brown, “Behind the Seven Veils, I: The Gnostic Life Setting of the Mystic Gospel of Mark,” in Ancient Gospel or Modern Forgery? The Secret Gospel of Mark in Debate, ed. Tony Burke (Eugene, Oregon: Wipf and Stock, 2013), 260.
 Anne Mary Farrell, “Plato’s Use of Eleusinian Mystery Motifs,” (PhD Dissertation, University of Texas, 1999), 38, 48-51.
 Burkert, Ancient Mystery Cults, 101; Guy G. Stromousa, Hidden Wisdom: Esoteric Traditions and the Roots of Christian Mysticism, 2d ed. (Leiden: Brill, 2005), 1, 98.
 Farrell, “Plato’s Use of Eleusinian Mystery Motifs,” 22-26.
 Burkert, Ancient Mystery Cults, 3, The possible connections between the mysteries and Christianity has been hotly debated for centuries. See Jonathan Z. Smith, Drudgery Divine: On the Comparison of Early Christianities and the Religions of Late Antiquity (University of Chicago Press, 1990).
 Farrell, “Plato’s Use of Eleusinian Mystery Motifs,” 57-59.