Calvin: I have exterminated Michael Servetus the Spaniard.

In a recent discussion, the following John Calvin quote was provided:

Letter to the Marquis Paet, chamberlain to the King of Navarre, 1561. "Honour, glory, and riches shall be the reward of your pains; but above all, do not fail to rid the country of those scoundrels [Anabaptists and others], who stir up the people to revolt against us. Such monsters should be exterminated, as I have exterminated Michael Servetus the Spaniard."

This quote has been around for quite awhile, receiving its popularity because of its negative reference to Michael Servetus, "I have exterminated Michael Servetus the Spaniard." Here, Calvin directly admits to having Servetus killed. This admission would be contrary to the popular defense of Calvin's involvement that he had no authority in Geneva to put anyone to death.   Here is not the place to delve into a defense or prosecution of Calvin's involvement in the execution of Servetus. What interested me is the actual letter this quote comes from and the possibility it may be a forgery.

Documentation
In the snippet above, the only documentation provided is that the quote is from a 1561 letter "to the Marquis Paet, chamberlain to the King of Navarre." If you search the first phrase of the snippet, it becomes obvious it's a blatant cut-and-paste. For example, you can find it in the exact form here and here. A more broad search demonstrates the quote was often used in the 1800's.  For instance,  in the 1848 book, The life of Michael Servetus, the author states,

No one could know the history of Calvin and Servetus better than Voltaire. He affirms that the former acted towards the latter with treachery and theological hatred; that "when he saw his adversary in confinement, he loaded him with every kind of insult and vile treatment that base minds are wont to do when they get the upper hand. At length, by continually pressing the judges to employ the credit of those he pointed out to them, and by proclaiming in person, and by his emissaries, that God demanded the execution of Michael Servetus, he had him burned alive, and took a cruel pleasure in being a witness to his sufferings; he who, if he had set a foot in France would have heen sent to the stake himself, and who had so loudly exclaimed against all persecution." The finishing stroke to this picture of Calvin may be found in a letter written with his own hand, which is still preserved in the castle of Bastie-Roland, near Montelimar: it is directed to the Marquis de Poet, high chamberlain to the king of Navarre, and dated Sept. 30, 1561. "Honour, glory, and riches shall be the reward of your pains; but above all, do not fail to rid the country of those zealous scoundrels who stir up the people to revolt against us. Such monsters should be exterminated as I have exterminated Michael Servetus the Spaniard."*

* Works of Voltaire translated by Smollet, and Franklin, &;c.—Lond. 1763. vol. iv. pp. 82—84.

This old book gives a reference to  the works of Voltaire vol. 4.  In the 1761 edition, Voltaire's comment can be found on page 84 as part of a chapter entitled "Calvin and Servetus":

It appears that Voltaire was the source for this popular quote, and the English translator provided the form that it's in. A contemporary English version of Voltaire's words can be found here. Voltaire had a severe view of Calvin (as this  biographical book documents). This letter from Calvin cited by Voltaire does indeed exist. It was included in Dr. Jules Bonnet's collection of Calvin's letters, published in French in the 1850's. There is an English translation of Calvin's letters from Bonnet's French. The letter in question can be found in Letters of John Calvin Vol. IV, pp. 439-440. There is a typo in the English translation. In the letter below, the date is said to be "8th September, 1561." The French version verifies that this is an error, the actual date should have been September 13, 1561.


Context

XVII.—To MONSEIGNEUR, MONSEIGNEUR DU POET, GRAND CHAMBERLAIN OF NAVARRE AND GOVERNOR OF THE TOWN OF MONTELIMART, AT CREST..

MONSEIGNEUR :—What have you judged of the Colloquy of Poissy? We have conducted our business safely. The Bishop of Valence as well as the others have signed our profession of faith. Let the king make processions as much as he pleases, he will not be able to hinder the preaching of our faith, harangues in public, nor gain anything except to stir up the people already too disposed for rebellion. The brave Seigneurs de Montbrun and de Beaumont abandon their opinions. You spare neither courses, nor cares; labour, you and yours will find their turn (sic). One day, honour, glory, and riches will be the reward of so much pains. Above all, do not fail to rid the country of all those zealous scoundrels that stir up the people by their discourses to make head against us, blacken our conduct, and wish to make our belief pass for a reverie. Such monsters should be smothered, as I have done here, by the execution of Michel Servetus the Spaniard. Do not imagine that in future any one will take it into his head to do the like. 

For the rest, Monseigneur, I forgot the subject for which I did myself the honour to write to you, which is humbly to kiss your hands, supplicating you to take in good part the quality which I shall covet during my whole life of . . . 

MONSEIGNEUR,
 Your very humble and affectionate servant, 

                                                                J. CALVIN.

At Geneva, this 8th September, 1561.
[Fr. Copy—Arch. of M. le Marquis d' Alissac a Valreas.]  

Conclusion 
Dr. Jules Bonnet included this letter in a chapter entitled, "An Historical Calumny Refuted." There Bonnet documents the spuriousness of the letter. He mentions that the first publication of the letter was in 1750, and indeed that Voltaire popularized the quote in question. He also documents that letter swept across nineteenth century scholarship. After personally seeing the letter, Bonnet argues it (along with another one) is a fraud. His reasons are as follows:

1st. These originals, written by Calvin’s own hand (as Voltaire affirms), are anything but autographs. They are neither in the handwriting of Calvin, nor in that of Jonvillers his secretary, nor of Antony Calvin, who sometimes held the pen under the dictation of the Reformer during the latter years of his life.

2nd. If these pieces are not in the handwriting of Calvin, still less do we find in them his style, admired by Bossuet himself and one of the finest in our language. That style is concise, nervous, and dignified, bearing the impress of a strong individuality more easy to caricature than to imitate.

3rd. From the form let us pass to the substance. The two letters swarm with mistakes and historical blunders which betray the work of an unskillful forger. The first, dated the 8th May, 1547, and addressed to M. du Poet, General of the Religion in Dauphiny, bestows this title on this seigneur, fifteen years before the period in which he declared for the Reformation, and when the new faith, having neither church nor soldier in Dauphiny, could scarcely enumerate some obscure martyrs in that province. The second, dated the 13th September, 1561, has for superscription — to M. du Poet, grand chamberlain of Navarre and Governor of Montelimart, dignities with which he was invested only twenty years later, in 1584. It is one of Calvin’s accusers, M. Aubenas himself, who informs us of that, without remarking that the notice which he has devoted to M. du Poet is the best refutation of the authenticity of the letters attributed to the Reformer.

The first reason given does not necessarily prove the letter is a forgery. I would assume Dr. Bonnet was familiar enough with the writing styles of the men he mentions, yet it does not rule out that Calvin could have dictated the letter to someone else as an exception. The second reason given is even less convincing. However, the third reason, of blunders with dates, is compelling. The alleged letter gives someone a title he did not hold until twenty years later... that is indeed a problem, and it's enough then to pull the letter from historical evidence until further notice.  I did a cursory search to see if any more recent Calvin scholarship discussed the letter, and did not come across anything, yet. When I mentioned in dialog that the letter is a suspected forgery, the response given back was "James I Don't believe you, both quotes are found in numerous places especially the quote about Servetus." But then again, this person thinks Calvin was a "demon possessed murderer" so it probably wouldn't matter if there were a dozen more reasons proving the letter spurious.