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.
The factors that led to the excommunication, heresy label, and death of this individual were his advocacy for vernacular Bible translations, his opposition to simony, and his belief in Purgatory.