During his work at universities in Leipzig, Chicago, Marburg and Santa Barbara, he acquired an international reputation as an expert in Gnosticism and Mandaeism. He also occupied himself with Islam and methodological questions in religious studies.
His priority was the creation of a religious studies discipline that was independent of theology. Rudolph stressed that religious studies must be a rational science and be subjected to methodological atheism. This theory, which was initially fiercely contested in German religious studies, is now largely a matter of consensus.