His teachings that the Bible should be translated into vernacular tongue (In this case Czech), his opposition to Simony (which was wide spread at the time) and the existence of Purgatory lead to him being excommunicated, deemed a heretic and burnt at the stake in 1415.
Because he advocated for translating the Bible into Czech, opposed simony, and believed in Purgatory, he was condemned as a heretic, excommunicated, and executed by burning in 1415.