Last Grand Master of Teutonic Order in Prussia, Albrecht Hohenzollern, converted to Lutheranism and secularised Prussian branch of the Order in 1525 and ruled the Duchy as vassal of his uncle Sigismund the Old, and then cousin Sigismund Augustus until 1568. Albrecht was ardent Lutheran and soon started to impose new faith upon his subjects. Largest towns, like Königsberg, were already centers of Reformation, but rural population was initially hardly affected by the spread of new faith. Large part of Prussian population was still pagan in fact. Albrecht with slight changes of liturgy, so peasants barely noticed, that they switched denomination, but over time Ducal Prussia was successfully converted.
So there is the POD-Albrecht's rule does not last long ITTL, he dies very next year after secularisation of the Order State, in 1526, leaving no children. According to treaty of Cracow from 1525 Albrecht's brothers had rights to succeede him if he dies without son. Oldest brother of Albrecht, Casimir Margrave of Kulmbach was still alive in 1526. He died IOTL from dysentery, fighting in Hungary on Habsburg side, so his death could be easily butterflied away-instead of going to Hungary he'd move to Prussia to inherit his brother's duchy. Casimir was power-hungry and ruthless ruler, as oldest of brothers he should be Albrecht's successor. What is worth to note-Kasimir, besides being cruel monster (he wasn't called "Bloodhound" for no reason) remained Catholic and opposed reformation in his lands (although he still wasn't able to stop spread of protestantism completly and had to tolerate it to some degree). Casimir would not be able to stop spread of Lutheranism in places like Königsberg, but obviously he would not try to convert Prussian peasants to new faith. As result religious landscape of Ducal Prussia should resemble that of Royal (West) Prussia-Protestant towns (Konigsberg being Ducal Prussia's Danzig analogue) with largely Catholic countryside-Ermland/Warmia, area in the middle of East Prussia, that until 1772 belonged to PLC, remained Catholic until 20th century, both German and Polish speaking population stayed Catholic, propably most of Ducal Prussia would resemble Ermland/Warmia ITTL. That means, for example, that there would be practically no difference between Polish speaking Masurians and Mazovians, also Lithuanian speakere, who still dominated northeastern part of Prussia, would share the same denomination with Lithuanians from Grand Duchy of Lithuania. Also, being Catholic and having Jagiellon blood, Prussian Hohenzollerns have good chance to get also Polish crown once Jagiellons died out in male line. And that is not everything-life of Albert Alcibiades and course of Schmalkaldic Wars also would be significantly affected...
So there is the POD-Albrecht's rule does not last long ITTL, he dies very next year after secularisation of the Order State, in 1526, leaving no children. According to treaty of Cracow from 1525 Albrecht's brothers had rights to succeede him if he dies without son. Oldest brother of Albrecht, Casimir Margrave of Kulmbach was still alive in 1526. He died IOTL from dysentery, fighting in Hungary on Habsburg side, so his death could be easily butterflied away-instead of going to Hungary he'd move to Prussia to inherit his brother's duchy. Casimir was power-hungry and ruthless ruler, as oldest of brothers he should be Albrecht's successor. What is worth to note-Kasimir, besides being cruel monster (he wasn't called "Bloodhound" for no reason) remained Catholic and opposed reformation in his lands (although he still wasn't able to stop spread of protestantism completly and had to tolerate it to some degree). Casimir would not be able to stop spread of Lutheranism in places like Königsberg, but obviously he would not try to convert Prussian peasants to new faith. As result religious landscape of Ducal Prussia should resemble that of Royal (West) Prussia-Protestant towns (Konigsberg being Ducal Prussia's Danzig analogue) with largely Catholic countryside-Ermland/Warmia, area in the middle of East Prussia, that until 1772 belonged to PLC, remained Catholic until 20th century, both German and Polish speaking population stayed Catholic, propably most of Ducal Prussia would resemble Ermland/Warmia ITTL. That means, for example, that there would be practically no difference between Polish speaking Masurians and Mazovians, also Lithuanian speakere, who still dominated northeastern part of Prussia, would share the same denomination with Lithuanians from Grand Duchy of Lithuania. Also, being Catholic and having Jagiellon blood, Prussian Hohenzollerns have good chance to get also Polish crown once Jagiellons died out in male line. And that is not everything-life of Albert Alcibiades and course of Schmalkaldic Wars also would be significantly affected...
