Castellio was born in 1515. He was educated at the University of Lyon and worked with John Calvin in Strasbourg and Geneva before doctrinal differences developed which led to his dismissal as rector of the College of Geneva in 1544. In 1553 he became professor of Greek at the University of Basel. Castellio died in ill health

Castellio published De haereticis (Concerning Heretics) in 1554, using the pseudonym Martinus Bellius. Concerning the execution of Michael Servetus, he wrote, "When Servetus fought with reasons and writings, he should have been repulsed by reasons and writings."

J. Frank Schulman, of the Unitarian Universalist Association, said that Castellio is probably "the first person in history who could be called a Unitarian in the modern sense of the word." He advocated religious tolerance, specifically contesting government expulsions of religious nonconformists.

Sébastien Castellion Sebastian Castellio 卡斯特利奧