In 1957, four years later, under the direction of the renowned professor of fundamental theology Gottlieb Söhngen, he qualified for University teaching with a dissertation on: "The Theology of History in St Bonaventure".
After lecturing on dogmatic and fundamental theology at the Higher School of Philosophy and Theology in Freising, he went on to teach at Bonn, from 1959 to1963; at Münster from 1963 to 1966 and at Tübingen from 1966 to 1969. During this last year he held the Chair of dogmatics and history of dogma at the University of Regensburg, where he was also Vice-President of the University.
From 1962 to 1965 he made a notable contribution to Vatican II as an "expert"; being present at the Council as theological advisor to Cardinal Joseph Frings, Archbishop of Cologne.
His intense scientific activity led him to important positions at the service of the German Bishops’ Conference and the International Theological Commission.
In 1972 together with Hans Urs von Balthasar, Henri de Lubac and other important theologians, he initiated the theological journal "Communio".