the faith they felt would infuse them with the strength necessary to resist the invader. They found this in the teaching of Protestant reformers such as Martin Luther. The spread of Protestantism in the country was assisted by its large ethnic German minority, which could understand and translate the writings of Martin Luther. While Lutheranism gained a foothold among the German- and Slovak-speaking populations, Calvinism became widely accepted among ethnic Hungarians. In the more independent northwest, the rulers and priests, protected now by the Habsburg monarchy, which had taken the field to fight the Turks, defended the old Catholic faith. They dragged the Protestants to prison and the stake wherever they could. Such strong measures only fanned the flames of protest, however. Leaders of the Protestants included Mátyás Dévai Bíró, Mihály Sztárai, István Szegedi Kis, and Ferenc Dávid. Protestants likely formed a majority of Hungary's population at the close of the 16th century, but Counter-Reformation efforts in the 17th century reconverted a majority of the kingdom to Catholicism. A significant Protestant minority remained, most of it adhering to the Calvinist faith. In 1558 the Transylvanian Diet of Turda decreed the free practice of both the Catholic and Lutheran religions, but prohibited Calvinism. Ten years later, in 1568, the Diet extended this freedom, declaring that "It is not allowed to anybody to intimidate anybody with captivity or expulsion for his religion". Four religions were declared to be "accepted" (recepta) religions (the fourth being Unitarianism, which became official in 1583 as the faith of the only Unitarian king, John II Sigismund Zápolya, r. 1540–1571), while Eastern Orthodox Christianity was "tolerated" (though the building of stone Orthodox churches was forbidden). During the Thirty Years' War, Royal (Habsburg) Hungary joined the Catholic side, until Transylvania joined the Protestant side. Between 1604 and 1711, there was a series of anti-Habsburg uprisings calling for equal rights and freedom for all Christian denominations, with varying success; the uprisings were usually organised from Transylvania. The Habsburg-sanctioned Counter-Reformation efforts in the 17th century reconverted the majority of the kingdom to Catholicism. The center of Protestant learning in Hungary has for some centuries been the University of Debrecen. Founded in 1538, the university was situated in an area of Eastern Hungary under Ottoman Turkish rule during the 1600s and 1700s, being allowed Islamic toleration and thus avoiding Counter-Reformation persecution. Romania Transylvania in what is today's Romania was a "dumping ground