Calixtus III issued this bull to institute special prayers to be said by Christians at a time of Turkish encroachments in the Balkans. It was part of the campaign of Calixtus to organize Europe into a crusade against the Turks, who had captured Constantinople in May 1453, and continued to advance into Europe. A copy of the Bull reached Mainz and was printed by Johann Gutenberg; only the present copy in the Scheide Library survives. A German translation was also printed by Gutenberg. It too survives in only one copy, in the Staatsbibliothek Preussischer Kulturbesitz, Berlin. Although no surviving example of early European printing is signed by Johann Gutenberg, early evidences and reports converge to show that he was the inventor of European typography. In particular, the “DK” (Donatus and Kalendar) type appears to be his first printing type. It was used in part to print the 31-line Cyprus Indulgence, of which the earliest datable copy, executed in Erfurt on 22 October 1454, is in the Scheide Library: this is the first fixed date at which we know that printing was being carried out in Mainz. Several other DK-type fragments, such as the Sibyllenbuch partial leaf at the Gutenberg Museum, Mainz, show a much less finished state of the font, and are plausibly earlier than 1454. In the late 1450s, the DK type was apparently sold to Bamberg, where it was used to print the 36-line Bible (not after 1461), and other books, some of which are signed by Albrecht Pfister.