From: Last Admonition of John Calvin to Joachim Westphal, Who, if He Heeds it Not, Must Henceforth be Treated in the Way Which Paul Prescribes for Obstinate Heretics (1557); in the source, Tracts Relating to the Reformation, Volume 2; Jean Calvin, Théodore de Bèze, Henry Beveridge (Calvin Translation Society, 1849), pp. 357-358 (bolding my own):
First, he maintains, that to prevent the contagion from spreading, sectaries and heretics are to be banished or otherwise subjected to punishment. As we are both agreed on that matter, all he had to do was to subscribe to us. . . .
Westphal, quoting exactly “Lift up your heads, O ye doors,” says, the passage enjoins magistrates to open the doors to the Lord, and shut them against false prophets. . . . the worthy man, in his eagerness to throw obloquy on me, was not ashamed to insert in the farrago, to which he gives the name of Confessions, the letter of some follower of Servetus, in which I am called an incendiary for having taught that heretics are justly punished. Let the letter be read. It brings no other charge against me than that I teach that rulers are armed with the sword not less to punish impiety than other crimes. The only difference between me and Westphal is, that I say there is no room for severity unless the case has been previously discussed. Nay, as it is usual with the Papists in the present day to inflict cruelties on the innocent without any investigation, I justly condemn the barbarity, and recommend that no severe measure be ever adopted until after due cognizance; and I carefully warn them against being too credulous, lest they may defile their hands by indiscriminate slaughter.
I then complain and lament that the world has been reduced to such slavery that no discussion takes place, and those who domineer under the name of prelates will not hear a word at variance with their decrees; nay, will not even allow doubt or inquiry. I say that it is barbarity not to be tolerated, when without cognizance mere possession, unsupported by right or reason, is maintained by the sword. Certainly as, according to an ancient saying, ignorance is audacious, so in this preposterous zeal cruelty is added to audacity. I therefore enjoin the true worshippers of God to take heed not rashly to undertake the defence of an unknown cause, nor be hurried by intemperance into severity; for as, in earthly causes, a judge who, himself in ignorance of the whole matter, lazily passes sentence on the opinion of others, is justly condemned; so, how much more deserving are judges of condemnation when, in the cause of piety, they, from disdain, omit to investigate?
And I have not taught in word any thing that I have not confirmed by act. For when Servetus was, by nefarious blasphemies, overthrowing whatever piety exists in the world, I, nevertheless, called him to discussion; and not only came prepared to give an account of my own doctrine, but chose rather to swallow the reproaches of that vilest of men, than furnish a bad example, by enabling any one afterwards to object that he was crushed without being heard. Westphal deems it enough for magistrates to oppose the sword in place of discussion; and it is no wonder that a man, whose only hope of victory is placed in darkness, should tyrannically rage while suppressing the light of truth.
Note that Calvin is not at all against capital punishment (as happened in the famous Servetus case); he is only opposed to the absence of discussion and attempted persuasion of heretics and a thorough, fair trial and sufficient evidence of guilt before punishing them (assuming they are unrepentant).