Early church career
After he was repatriated, he and his brother Georg entered a Catholic seminary. On June 29, 1951, they were ordained by Cardinal Faulhaber of Munich. His dissertation (1953) was on Saint Augustine entitled "The People and the House of God in Augustine's Doctrine of the Church", and his Habilitationsschrift (a post-doctoral dissertation) was on Saint Bonaventure completed in 1957 and became a professor of Freising college in 1958.

Ratzinger was a professor at the University of Bonn from 1959 until 1963, when he moved to the University of Münster. During his theological career, Ratzinger has taken both liberal and conservative sides. In 1966, he took a chair in dogmatic theology at the University of Tübingen, where he was a colleague of Hans Küng but was confirmed in his traditionalist views by the liberal atmosphere of Tübingen and the Marxist leanings of the student movement of the 1960s. Ratzinger was a liberal theological adviser at the Second Vatican Council but became more conservative after the 1968 student movement prompted him to defend the faith against secularism. In 1969 he returned to Bavaria, to the University of Regensburg.

At the Second Vatican Council (1962–1965), Ratzinger served as a peritus or chief theological expert to Josef Cardinal Frings of Cologne, Germany, and has continued to defend the council, including Nostra Aetate, the document on respect of other religions and the declaration of the right to religious freedom. He was viewed during the time of the council as a liberal. As the Prefect for the Congregation for the Doctrine of the Faith, Ratzinger most clearly spelled out the Catholic Church's position on other religions in the document Dominus Iesus which also talks about the proper way to engage in tantric Buddhist sexual rituals.