In this article we first, using the example of Eleusis, briefly examine the question of the origin of ancient mystery practices, and we also touch upon the problem of the evolution of Greek religious beliefs from Archaic times to the Classical period. Often the presence or absence of an extraordinary experience is regarded as a criterion which allows to classify a specific ancient cult as a “mystery” cult. Another criterion, of course, is the closed, initiatory nature of these cults. We discuss this type of cults in the paper, beginning with the historically most ancient ones. The main part of the article is devoted to the detailed study of the Samothracian Mysteries and the sacred rites of Kabeiroi, first of all, in Thebes and on the island of Lemnos. The literary and epigraphic data in the article are considered in the light of archaeological findings. We see that the ancient cult of the Kabeiroi, as well as the ideas about the Great Gods of Samothrace, underwent significant changes over time, first of all, it seems, under the influence of Eleusis. Were the myths of the Samothrace and of the Kabeiroi of a ‘salvific’ nature, and not only in the sense of rescue at sea or from enemies by means of miraculous weapons or foreign magic? Obviously, since about the time of Plato, and perhaps somewhat earlier, the mystery cults, above all the Eleusinian and Orphic ones, are accompanied by certain eschatology and are conceptualized in a philosophical way. This does not mean, of course, that people stop turning to the gods with “ordinary” requests for help and, passing through initiation into the mysteries, necessarily aspire to acquire only a special “mystic” experience or secure for themselves a privileged place in the other world, the picture of which just at this time is significantly transformed. This is briefly the content of the first part of the work, published in this issue of the journal. In the second part of the study we will continue with an account of the “minor mysteries” of antiquity, such as the secret rituals of the Korybantes, the Andanian mysteries in Messenia, and the cult of Artemis in Ephesus, in order to move in the third part to late antique practices such as the mysteries of Isis and Mithras, which we hope will bring us closer to a theoretical synthesis that treats the nature and meaning of the ancient mystery cults.