Assembly) stated that 'everybody should live in a belief that he or she wants if it is done without the distrust of another' which meant for the population of the Principality of Transylvania that it became allowed to practise not just the Roman Catholic, but the Lutheran religion. Calvinism In 1559 he entered the Reformed Church where he was elected bishop of the Hungarian churches in Transylvania and he was also the appointed court preacher to János Zsigmond Zápolya, Prince of Transylvania. The prince allowed him to research in the royal library and to work in the royal court on his theological theses. Unitarianism After the Battle of Mohács the political instability, the weakening of the Roman Catholic denomination (continuous expansion of the Ottoman Empire, heretic movements in Transylvania especially of Arianism, Bogumilism etc.) prepared the way for the new ideas of the Reformation. A well known Italian antitrinitarian, Giorgio Biandrata moved to Transylvania in 1563 into the royal court of John II Sigismund Zápolya and became his own doctor. Biandrata co-operated with Ferenc Dávid on theological works. Dávid's discussion of the Holy Trinity began in 1565, with doubts of the personality of the Holy Spirit, because he could find no scriptural basis for the doctrine of the Trinity. One of his main points against the existence of the Holy Trinity was that what the Arians during the early ages of Christianity liked to refer to does not come up in the Bible. He was influenced by the antitrinitarian and humanist views of Michael Servetus and Giovanni Valentino Gentile. Together with Giorgio Biandrata he published polemical writings against Trinitarian belief, particularly De falsa et vera unius Dei Patris, Filii et Spiritus Sancti cognitione which is largely a summarized version of Servetus's Christianismi Restitutio. But in 1578 the collaboration broke up as Biandrata was charged with immorality. An important difference between the views of the two theologians was that Ferenc Dávid became a nonadorant which meant that he renounced the necessity of invoking Christ in prayers. Working in the royal court, he convinced the prince about his point of view on religion, so that John II Sigismund Zápolya accepted his theses and became the first Unitarian ruler. In 1567 John II Sigismund Zápolya allowed him to use his press in Gyulafehérvár (today Alba Iulia, Romania) to propagate the religion. Edict of Torda (1568) The aim of his life as Ferenc Dávid