In 1966, Ratzinger was appointed to a chair in dogmatic theology at the University of Tübingen, where he was a colleague of Hans Küng. In his 1968 book "Introduction to Christianity", he wrote that the pope has a duty to hear differing voices within the Church before making a decision, and he downplayed the centrality of the papacy. During this time, he distanced himself from the atmosphere of Tübingen and the Marxist leanings of the student movement of the 1960s that quickly radicalised, in the years 1967 and 1968, culminating in a series of disturbances and riots in April and May 1968. Ratzinger came increasingly to see these and associated developments (such as decreasing respect for authority among his students) as connected to a departure from traditional Catholic teachings. Despite his reformist bent, his views increasingly came to contrast with the liberal ideas gaining currency in theological circles. He was invited by Rev. Theodore Hesburgh to join the theology faculty at the University of Notre Dame, but declined on grounds that his English was not good enough.