Protestant ideas were first introduced to France during the reign of Francis I of France (1515–1547) in the form of Lutheranism, the teachings of Martin Luther. Discussion and written works circulated in Paris unimpeded for more than a year. Although Francis firmly opposed Lutheranism as being heresy, the initial difficulty was in recognizing precisely what was heretical and what was not. Roman Catholic doctrine and the definitions of its orthodox beliefs were unclear. Francis tried to steer a middle course in the developing religious schism in France. Despite this, in January 1535, Catholic authorities decided that those classified as "Lutherans" were actually Zwinglians (also heretical), followers of Huldrych Zwingli. Calvinism, another form of Protestant religion, was soon introduced by John Calvin, a native of Noyon, Picardy, who fled France in 1535 after the Affair of the Placards.