Wi Sigismund Vasa was a Protestant?

So there was a large Protestant population, which was officially protected by law (the Warsaw Confederation). There were many protestant nobles. But Sigismund OTL was a champion of the Counter-Reformation.

So my questions are: Did the Polish constitution demand a Catholic King?

If Sigismund publicly converted to Protestantism, what kind of a response would that ellicit from the Polish nobility?
 
So my questions are: Did the Polish constitution demand a Catholic King?

I think so. Friedrich August III had to convert to Catholicism. Also, Wikipedia page on the coronation of Polish King and their wifes:

Kings and queens not crowned: [...]
Helena of Moscow, wife of Alexander Jagiellon, daughter of Ivan III, Grand Duke of Moscow, because she was of Orthodox faith
Christiane Eberhardine of Brandenburg-Bayreuth, wife of August II the Strong, daughter of Christian Ernst, Margrave of Brandenburg-Bayreuth, because she was of Protestant faith
 
Originally posted by Matthais Corvinus
So my questions are: Did the Polish constitution demand a Catholic King?
I couldn't find any article clearly establishing that the king had to be a catholic, but since noblemen were mostly catholic, chances for election of a non-catholic candidate were slim. Also, an interrex (acting king between death of a king and election of his successor) was always catholic primate of Poland. However, at least one non-catholic tried to gain Polish crown. He was rejected not because he was an orthodox, but because he was Ivan the Terrible.

Helena of Moscow, wife of Alexander Jagiellon, daughter of Ivan III, Grand Duke of Moscow, because she was of Orthodox faith
Christiane Eberhardine of Brandenburg-Bayreuth, wife of August II the Strong, daughter of Christian Ernst, Margrave of Brandenburg-Bayreuth, because she was of Protestant faith

We're talking different times here. Helena lived before the Reformation, Christiane long after triumph of Counter-Reformation in PLC.
 
Top