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WI Poland, like Hungary, maintained a significant Protestant population to the current day? Poland today is a bulwark of Roman Catholicism, perhaps to fanaticism at times. Protestant communities exist, but are very small in number. What would it take to sustain a larger historical Lutheran and/or Calvinist presence in present day Polish borders? Redrawing the WW II-Polish borders to include Lutheran Germans doesn't count.
Perhaps the greatest challenge is to butterfly away the Jesuit counterreformation. Another challenge would be to Protestantize a sizeable hunk of the 16th century Polish nobility and maintain a high level of Protestantization among the higher classes. [1], [2] Also plausible is a "milder Reformation" situation: maintain Roman Catholic liturgy but incorporate Protestant theological elements within a "Church of Poland" independent of Rome. I could particularly see a Catholic-Lutheran hybrid not unlike the Scandinavian reformation, particularly Sweden.
A friend of mine once remarked that the (sometimes) hyper-Catholicism of the Poles has resulted from their being thown back and forth politically between Protestant Prussia and Orthodox Russia. I think that his observation that Catholicity maintained Polish identity throughout the centuries is apt, though troubled in many respects.