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 produced notes he wrote that openly criticized the law, stating that only those that 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.
He was arrested and beheaded on July 26, 1547.