Servetus was largely interested in pharmacology and humanism. He also conducted a number of meteorological and astronomical observations during his life. He also attempted to give one of the first accurate accounts of the human circulatory system. This caused tensions with the Church because it conflicted with long-standing theological accounts of the circulatory system, and because Servetus wanted to conduct autopsies, which was considered to be defilement by many at the time.
Servetus was also interested in theology. He opened correspondence with John Calvin in which he disputed the sacred and central doctrine of the Holy Trinity. As news of Servetus and his heresies spread, the Church became involved.
In 1553, Servetus was denounced as a heretic while in Vienna by one of Calvin's followers who had been offended by the humanistic doctrines of the physician. Servetus was imprisoned and found guilty of heresy by the French Inquisition, and sentenced to death by being burnt alive along with his books.
Servetus was able to escape from prison for a time, although his books were consigned to the purifying effigy which the Inquisition had built to cleanse the world of his doctrines, which were considered to be blasphemous. However, a short time later, he was apprehended in Geneva and, on October 27, 1553, was burned at the stake for the preaching of non-trinitarianism and anti-paedobaptism. For the crime of suggesting that children should not be baptized, Calvin had demanded that Servetus merely be beheaded, rather than burnt alive. Martin Luther helped see to it that the sale and printing of the Michael Servetus' books was immediately halted.
Despite Servetus's execution, his teachings had already spread through the Libertine movement, thus setting in motion the events that gave rise to the modernist philosophy of Secular Humanism. He was also considered the first Unitarian martyr, although his theology has little to do with current Unitarian Universalism.