He did not regard the Ashkenazi Jews of Europe as being a Semitic people; Renan is credited with launching the so-called Khazar theory. This theory states that Ashkenazim had their origin in Turkic refugees that had converted to Judaism and later migrated from the collapsed Khazar Khanate westward into the Rhineland, and exchanged their native Khazar language for the Yiddish language while continuing to practice the Jewish religion. In his 1883 lecture "Le Judaïsme comme race et comme religion" he disputed the concept that Jewish people constitute a unified racial entity in a "biological" sense, which made his views unpalatable within racial antisemitism. Renan was also known for being a strong critic of German ethnic nationalism, with its antisemitic undertones. His notions of race and ethnicity were completely at odds with the European antisemitism of the 19th and 20th centuries.