Against Calvin's Booklet, in its full form Against Calvin's Booklet in which he attempts to show that heretics must be suppressed by the right of the sword (in Latin: Contra libellum Calvini in quo ostendere conatur haereticos jure gladii coercendos esse), is a theological treatise in the form of a dialogue written by Sebastian Castellio in June 1554 and published posthumously in 1612.

Castellio was a humanist Protestant theologian who started in Strasbourg, Lyon and then moved to Geneva to assist John Calvin.

[1][2] This opposition reached its peak with the execution of Michael Servetus, a Protestant theologian perceived as heterodox, who was burned at the stake in Geneva.

[1][2][3] Castellio wrote Against Calvin's Booklet shortly after the Treatise on Heretics, which explains the ideological proximity between the two works.

[7] Like the Treatise on Heretics, it is a "declaration of war" against Geneva and Calvin, finalizing the split between the two former colleagues.

[9] It seems that Castellio is strongly opposed to the death penalty in all cases, which might categorize him, perhaps somewhat anachronistically, as an abolitionist.