Gebhard Truchsess von Waldburg (10 November 1547 – 21 May 1601) was the prince-elector and archbishop of Cologne. After pursuing an ecclesiastical career, he won a close election in the Cathedral chapter of Cologne over Ernst of Bavaria. After his election, he fell in love with and later married Agnes von Mansfeld-Eisleben, a Protestant Canoness at the Abbey of Gerresheim. His conversion to Calvinism and announcement of religious parity in the Electorate triggered a war.
On 19 December 1582, a proclamation in his name established parity for Catholics and Calvinists in the Electorate of Cologne, causing a scandal in the Catholic Church and the Holy Roman Empire, and after his marriage in February 1583, he sought to convert the Electorate into a dynastic dignity. For the next six years, his supporters fought those of the Catholic cathedral chapter for the right to hold the electorship and the archdiocese in the so-called Cologne War or Seneschal War. After brutal fighting, plundering of villages, cities, and abbeys throughout the Electorate, Gebhard surrendered his claim on the electorate and retired to Strasbourg. He died there in 1601 and was buried in the Cathedral.
Gehard's conversion and marriage was the first major test of the principle of ecclesiastical reservation established in the Peace of Augsburg, 1555. His loss of the Electorate strengthened the Catholic counter reformation in the northern German states, gave the Jesuits a stronghold in Cologne, and expanded the Wittelsbach family influence in imperial politics.