After studying theology in Paris for the diocese of Basel in 1940, he was ordained a priest and worked for several years as a pastor in Lucerne. In 1942 Haag attained his PhD at the University of Fribourg. From 1948 to 1960 he taught Old Testament studies at the Theological Faculty of Lucerne, and from 1960 to 1980 he held the chair of Old Testament at the Catholic Theological Faculty at the University of Tübingen. Haag in particular as biblical scholars and exegete known, among other things, he published a well-known Bible dictionary. In his 1969 book on Farewell to the devil : from the Christian dealing with evil, he is noted for being the first Catholic theologian in the modern era to deny the existence of the devil, and to deny it as constitutive of the Christian faith, claiming it to be merely linked to a cultural frame inherited from both Judaism and paganism, a position which was criticized by then Cardinal Ratzinger. He also criticized dogmas of the church doctrine, such as original sin, apostolic succession, homosexuality, celibacy of the clergy and the ban on the ordination of women. In his last years he emerged as a critic of the institutional church.
In 1981, he signed an open letter to Swiss newspapers contending that a wrong had been done to Hans Küng in 1979, when a Vatican decree stripped the church's recognition of Küng as a Catholic theologian due to his critique of the church. In 1985, he established the Herbert Haag Foundation for "freedom in the Church", which awards the Herbert Haag Prize.
- editor, Bibel-Lexikon. 2nd edition. Einsiedeln and Zurich: Benziger, 1968.
- Biblische Schöpfungslehre und kirchliche Erbsündenlehre. Stuttgarter Bibelstudien 10. Stuttgart: Katholisches Bibelwerk, 1966; 4th edition 1968. English translation: Is Original Sin in Scripture? (1969).
- Der Gottesknecht bei Deuterojesaja. Erträge der Forschung 233. Darmstadt: Wissenschaftliche Buchgesellschaft, 1985. ISBN 3-534-09045-4.
- with Katharina Elliger, "Stört nicht die Liebe": Die Diskriminierung der Sexualität – ein Verrat an der Bibel. Olten, Switzerland, and Freiburg im Breisgau, Germany: Walter, 1986. ISBN 3-530-30101-9.
- Abschied vom Teufel. Einsiedeln: Benziger, 1969; new edition, Zurich: Benziger, 2000. ISBN 3-545-70016-X.
- Nur wer sich ändert, bleibt sich treu: Für eine neue Verfassung der katholischen Kirche. Freiburg im Breisgau: Herder, 2000. ISBN 3451268914.
- 2000:Prix Courage
- 2001:Pin of Lucerne
- Ratzinger, Joseph; XVI, Pope Benedict (2010-07-08). The Ratzinger Reader: Mapping a Theological Journey. A&C Black. ISBN 9780567032140.Theological foundations, p. 72.
- Pace, Eric (August 30, 2001). "Herbert Haag, 86, Priest Who Challenged Vatican". The New York Times.