In this short paper I try to outline how Theodoret of Cyrus did use in his work the term μυστήριον. This word has an extended semantic field. It can refer (among others) to any mystery or secret, to a mystery- or secret doctrine, to mystic implements, to divine secrets, etc. In the Late Antiquity it gained a semantic expansion, denoting the holiest rituals of the more and more consolidating Christianity. In the early Christian writings it can denote (1) a secret rite, (2) the oath of secrecy, (3) a sacred object, (4) a secret, (5) an object or an event of mystical significance and (6) the sacrament. A careful analysis of the occurrences of this word in the works of Theodoret can demonstrate that the bishop of Cyrrhus (Cyrus) has used the word µυστήριον in multiple meanings.