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.

This volume contains selected papers from the broader field of History of Christian Doctrines, and more specifically from Patristics and Reformation studies, but also from the border between them. Most of them were written in the academic year 2014-2015, during my MA studies in the field of Patristics at the Theological University of the (vrijgemaakt) Reformed Churches in the Netherlands, in Kampen. The main topic of the MA programme was the Baptism in the Early Church, therefore the most of these papers presents various aspects of this question.