Early years, education and career as theological scholar
Lehmann was born in Sigmaringen and grew up in Veringenstadt. His father was a local teacher and his mother educated as a bookseller. During his high school years, he lived in the Catholic student home
Erzbischöfliches Studienheim St. Fidelis in Sigmaringen.
He studied at the Seminary of Freiburg 1957–1964 and then at the Pontifical Gregorian University in Rome where he earned a doctorate in philosophy, in 1962, with a thesis titled Vom Ursprung und Sinn der Seinsfrage im Denken Martin Heideggers. He also received a doctorate in theology in 1967, with a thesis titled "Auferweckt am dritten Tag nach der Schrift"—Exegetische und fundamentaltheologische Studien zu 1 Kor 15, 3b-5.
He was ordained to the priesthood on 10 October 1963 in Rome by Cardinal Julius Döpfner. During the Second Vatican Council from 1962 to 1965, he served as an assistant to Karl Rahner, and he was also assistant to Rahner at the Seminar for Christian worldview and philosophy of religion at the Ludwig Maximilian University of Munich from 1964 to 1967.
In 1968 he became professor of dogmatic theology at the University of Mainz, a position he held until 1971 when he became professor of dogmatic and ecumenical theology at the University of Freiburg. 
He was for ten years, from 1974 to 1984, a member of the International Theological Commission.