German royalty, a Prince-elector of the Margraviate of Brandenburg from the House of Hohenzollern who also served as a Duke of Prussia.
John Sigismund was born to Joachim Frederick, Elector of Brandenburg, and his first wife Catherine of Brandenburg-Küstrin. He succeeded his father as Margrave of Brandenburg in 1608. In 1611, John Sigismund traveled from Köningsberg 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. Albert Frederick died in the following year.
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.
John Sigismund’s most significant action was his conversion from Lutheranism to Calvinism, after he had earlier equalized the rights of Catholics and Protestants in the Duchy of Prussia under pressure from the King of Poland. 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.
On 30 October 1594, John Sigismund married Anna of Prussia, daughter of Albert Frederick, Duke of Prussia (1553–1618). They were parents to eight children: George William (13 November 1595 – 1 December 1640), his successor; Anne Sophia of Brandenburg (15 March 1598 – 19 December 1659); Maria Eleonora of Brandenburg (11 November 1599 – 28 March 1655); Catherine of Brandenburg (28 May 1602 – 27 August 1644); Joachim Sigismund of Brandenburg (25 July 1603 – 22 February 1625); Agnes of Brandenburg (31 August 1606 – 12 March 1607); John Frederick of Brandenburg (18 August 1607 – 1 March 1608); and Albrecht Christian of Brandenburg (7–14 March 1609).
John Sigismund of Brandenburg died on 23 December 1619.
Link to Wikipedia biography