Poland had a significant Protestant population of burgers and lower nobles. Germans were often Lutheran, but the Lutheran religion was detested in Poland. Calvinism was preferred owing to the close relationship Poland had with France. The Polish King's certainly worked to decrease the influence of the Church; they ceased to pay Peter's Pence, for instance, and Sejm passed a Patent of Toleration. One interesting situation is Sigismund II Augustus. He married three times. First to Elizabeth of Austria, secondly to Barbara Radizwill, and thirdly to Catherine of Austria, his first wife's sister. After a miscarriage, Sigismund thought the marriage was cursed and she returned to Austria. He even tried to have the marriage annulled, the Habsburg influence within the curia insured it was impossible. Him having a legitimate heir was of great importance to Catholics and Protestants too, but he was too pious. Had he been a little less so, he might of used the same argument Henry VIII had used against Catherine of Aragon. He was closely connected to both the Catholic and Protestant parties, and managed to balance them both without angering them.
Another idea is just demography. Poland was a "haven" for Jews. So why not oppressed Protestants? It's something I plan to do in my TL, with Anabaptists and other radical sects seeking freedom in Poland, with the German communities embracing Lutheranism while Calvinism is popular amongst the Poles, with a growing community around Vilinius. Homegrown sects possible too, like the Polish Brethen, Arians, Antitrinitarians, ect.
