The chapter examines Beethoven’s encounter with the Catholic Enlightenment in the lands of the Bohemian crown, where his music had been widely known as early as the 1790s. Since the eighteenth century, linguistic issues, as championed by Bohemia’s Catholic Enlightenment, were key to addressing local questions of nationality and societal progress. Central to these debates were the teachings of Bernhard Bolzano. A priest, philosopher and theologian, but also a respected mathematician and astronomer, Bolzano wanted Catholicism to be compatible with the demands of reason. Although his radical thought was dismissed by the government, his ideas remained a source of inspiration for societal reforms when Beethoven’s famous pupil and patron archduke Rudolph became prince-bishop of Olomouc/Olmütz. For Beethoven, Rudolph’s new role was an important point of connection to the Catholic Enlightenment in the region. Among the many works Beethoven dedicated to the archduke was the Missa solemnis, op. 123, where he presented the world with a musical response to philosophical questions that had occupied him for decades and that also were at the core of the Catholic Enlightenment in Bohemia.