This first chapter is based on the text of a handout prepared for the very first lecture I gave at the newly founded Institute of Ecumenical Studies in Prague in 1995. The lecture was part of a course entitled “Introduction to Theological Thought”. I have decided to leave in the text the traces of the radicalism of my youth. I was 28 when it was written and the churches in the Czech Republic were, with much difficulty, emerging from the totalitarian mentality. Ecumenism, if it was to be more than formal belonging to the Ecumenical Council of Churches, was not welcome. A number of church representatives saw active and lived inter-church relations as a threat to the identity of their denomination.1 The decision to open the book with this particular text is not only because it is the very first of my studies which explicitly deals with the subject of Ecumenical Theology, but also because it stated a programme for Ecumenical Theology which I still find valid. The original title of the Czech text was “The Significance of Ecumenical Theology for the Life of the Christian and the Church”. It appeared in the textbook for students published by the Institute in 1996.2 The text has been translated into English by Tim Noble. This version also includes parts of the Introduction to the textbook, where the very concept of Ecumenical Theology is defined.3 The introductory section of the text and the subheadings have been substantially changed, but the rest of the text remains almost unaltered.