Henry Bullinger, in defense of a work of John Calvin on punishing heretics, wrote to him the following words of encouragement—including agreement with Calvin's support for the execution of Servetus, who was guilty of vile blasphemies:
I know that many have wished that you had not defended this principle; but many also thank you, and among others our church. Urbanus Regius has long ago proved, in a work of his own, and all the ministers of Luneberg agree with him, that heretics, when they are blasphemers, ought to be punished. There are also many other pious men who think the same, and consider that such offenders ought not only to be silenced, but to be put to death. Do not repent therefore of what you have done: the Lord will uphold your righteous efforts. I know that your disposition is not cruel, and that you will favour no barbarity. Who knows not, that a boundary must be set to things of this kind? But how it could be possible to spare such a man as Servetus, that serpent of all heresies, that most obdurate of men, I see not.
 Cited in Paul Henry, The Life and Times of John Calvin, the Great Reformer: Volume II, trans. Henry Stebbing (London: Whittaker and Co., 1849), 234.