Gruet used to frequent taverns, and his behaviour was unacceptable by the religious standards of those days. He wrote blasphemous notes and defied social conventions.
Gruet left an anonymous placard threatening Calvin, and the authorities investigated. A search of Gruet's house revealed some notes in which he openly criticized the law, stating that only those which go against the state should be upheld, and also depicting Calvin as a hypocrite. He also mocked the scriptures, ridiculed Christ and derided the immortality of the soul as a simple fairy tale.
In many ways, Jacques Gruet was ahead of his time in his thinking. Most of his ideas regarding the role of religion and the extent of freedom are practised today, at least in the Western world.
He was arrested by Calvin, tortured for a month and burned at the stake on July 26, 1547.