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.