He sparked a controversy on Islam when, during a conference in Regensburg, he quoted a passage from the text of a Byzantine ruler, Manuel II Palaeologus.
The Regensburg lecture was delivered on 12 September 2006 by Pope Benedict XVI at the University of Regensburg in Germany, where he had once served as a professor of theology. It was entitled "Faith, Reason and the University — Memories and Reflections" (German: Glaube, Vernunft und Universität — Erinnerungen und Reflexionen). The lecture is considered to be among the most important papal statements on world affairs since John Paul II's 1995 address to the United Nations, and sparked international reactions and controversy.
In his lecture, the Pope, speaking in German, quoted an unfavorable remark about Islam made at the end of the 14th century by Manuel II Palaiologos, the Byzantine emperor. As the English translation of the Pope's lecture was disseminated across the world, many Islamic politicians and religious leaders protested against what they saw as an insulting mischaracterization of Islam.
Mass street protests were mounted in many Islamic countries, the Majlis-e-Shoora (Pakistani parliament) unanimously called on the Pope to retract "this objectionable statement". The Pope maintained that the comment he had quoted did not reflect his own views, and he offered an apology to Muslims.
The controversial comment originally appeared in the 7th of the 26 Dialogues Held With A Certain Persian, the Worthy Mouterizes, in Anakara of Galatia, written in 1391 as an expression of the views of the Byzantine emperor Manuel II Palaiologos, one of the last Christian rulers before the Fall of Constantinople to the Muslim Ottoman Empire, on such issues as forced conversion, holy war, and the relationship between faith and reason. The passage, in the English translation published by the Vatican, is as follows:
“ Show me just what Muhammad brought that was new and there you will find things only evil and inhuman, such as his command to spread by the sword the faith he preached. ”