Could this really be true? There is no footnote to the passage but Barzun is an exemplary scholar. His comment is in the context of a discussion of the volume of communication, discussion and writing attendant to the Reformation.
From his unexpected sabbatical onward, Luther kept addressing the Germans on every issue of religious, moral, political, and social importance. Pamphlets, books, letters to individuals that were "given to the press" by the recipients, biblical commentaries, sermons, and hymns kept streaming from his inkwell. Disciples made Latin translations of what was in German and vice versa. It was an unexampled barrage of propaganda to pose a countrywide issue. Opponents retorted, confrontations were staged at universities and written up. A torrent of black-on-white wordage about the true faith and the good society poured over Christian heads. It did not cease for 350 years: 1900 was the first year in which religious works (at least in England) did not outnumber all other publications.