John Sigismund was a Prince-elector of the Margraviate of Brandenburg from the House of Hohenzollern.
Background
He became the Duke of Prussia through his marriage to Duchess Anna, the eldest daughter of Duke Albert Frederick of Prussia who died without sons. John Sigismund was born in Halle an der Saale to Joachim III Frederick, Elector of Brandenburg, and his first wife Catherine of Brandenburg-Küstrin. He succeeded his father as Margrave of Brandenburg in 1608.
Education
John Sigismund gave the Reichshof Castrop to his teacher and educator Carl Friedrich von Bordelius, whereas he received the territories of Cleves, Mark, and Ravensberg in the Treaty of Xanten in 1614.
Career
Their marriage resulted in the creation of Brandenburg-Prussia. In 1611, John Sigismund traveled from Königsberg to Warsaw, where on 16 November 1611 he gave feudal homage to Sigismund III Vasa, King of Poland (the Duchy of Prussia was a Polish fief at the time). He officially became Duke of Prussia in 1618, although he had served as regent on behalf of the mentally-disturbed Albert Frederick, Duke of Prussia, for several years prior.
John Sigismund died in the following year.
Resistance was so strong that in 1615, John Sigismund backed down and relinquished all attempts at forcible conversion. Instead, he allowed his subjects to be either Lutheran or Calvinist according to the dictates of their own consciences.
Henceforward, Brandenburg-Prussia would be a bi-confessional state.