The Rhine Province changed Prussia's religious structure considerably. The Rhenish-Bergisch Reformed Church strengthened Calvinism in that Lutheran bastion. Like them, the "Old Lutherans" around the Reverend Claus Harms (1778-1855) from Kiel, and orthodox and pious Lutherans in Silesia rejected Church Union, because in the course of State integration and balancing out regional interests, they feared for the identity of their Christian denomination. The annexation of the Rhineland nearly doubled the share of Catholics to over forty percent of a total of 10.4 million inhabitants in Prussia. This fact did not escape the Prussian State nor the Roman Curia. After the Catholic Church had lost political power and feudal property in western Germany during the time of secularization, the Papal See intensified its dogma rigorously and subsequently came into conflict with the State.
What triggered the „Mischehen" (intermarriage) dispute between the Prussian government and the Roman Curia was the transfer of thousands of Protestant civil servants to the Rhine, in order to set up the Prussian administration. However, the deeper causes derived from the Enlightenment, its State philosophy and criticism of religion. While Berlin wanted to assert its State sovereignty also in questions of religion, Rome insisted on its official sovereignty in matters of faith. Moreover, it not only opposed the competing claims of the State, but also fought against inner-church rivals for its teaching authority. The "Cologne confusion" and the case of the Bonn theologian Georg Hermes (1775-1831), whose objective-scientific teaching of revelation cost him his job, were evidence of the great influence which the French Enlightenment exerted in the Rhineland, not only on the relationship between Church and State, but also within the (here: Catholic) Church.