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In page German Empire:

"

Confessional demographics of the early modern period had hardly changed. There remained almost entirely Catholic areas such as Altbayern and almost entirely Protestant areas (Schleswig-Holstein, Pomerania, Saxony, etc.). Confessional prejudices were common until secularization and internal migration broke down the old barriers. In the East, confession was almost uniquely perceived to be connected to one's ethnicity, and the equation "Protestant = German, Catholic = Polish" was held to be valid. In areas affected by immigration such as NRW, the confessional landscape changed substantially through mass Protestant immigration.[citation needed]