Sebastian Castellio (also Sébastien Châteillon, Châtaillon, Castellión, and Castello; 1515 – 29 December 1563) was a French preacher and theologian; and one of the first Reformed Christian proponents of religious toleration, freedom of conscience and thought.

"[2][3][4][5] Castellio later wrote that he was deeply affected and moved when he saw the burning of heretics in Lyon by the French Inquisition,[6] and at the age of twenty-four he decided to subscribe to the teachings of the Reformation.

[7] For his outstanding work, the Geneva City Council recommended Castellio's permanent appointment as preacher in Vandoeuvres; however in 1544 a campaign against him was initiated by Calvin.

Calvin wrote to a friend regarding the matter: "Just listen to Sebastian's preposterous scheme, which makes me smile, and at the same time angers me.

Anticipating future attacks from Calvin, Castellio asked for a signed letter that outlined in detail the reasons for his departure: "That no one may form a false idea of the reasons for the departure of Sebastian Castellio, we all declare that he has voluntarily resigned his position as rector at the College, and up till now performed his duties in such a way that we regarded him worthy to become one of our preachers.

[11] Castellio's existence ranged from begging and digging ditches for food to proof-reading for the Basel printshop of Johannes Oporinus.

[13] Castellio's fortunes gradually improved, and in August 1553 he was made a Master of Arts of the University of Basel and appointed to a prestigious teaching position.

[15] However, in October 1553, the physician and theologian Michael Servetus was executed in Geneva for blasphemy and heresy – in particular, his repudiation of the doctrine of the Trinity.

"[16] However, many other contemporary scholars, such as David Joris and Bernardino Ochino (also known as the remonstrators) were outraged both publicly and privately over the execution of Servetus.

Three months later, Castellio wrote (as Basil Montfort) a large part of the pamphlet Treatise on Heretics (De haereticis, an sint persequendi) with the place of publication being given on the first page as Magdeburg rather than Basel.