"Perhaps the most widely-known example of despair in the sixteenth and seventeenth century was the case of an Italian lawyer Francis Spira. In 1548, Spira converted to Lutheranism and began to spread the Lutheran message to others. Under pressure from the Catholic Church, however, he renounced his Protestant faith. He then became convinced that he was a reprobate, destined for hell. The story of Spira spread throughout Europe, surfacing in sermons and treatises dealing with despair. In England, an account of Spira's case by a first-hand witness, Matteo Gribalde, appeared in 1550. The most influential English account of Spira, however, was written by Nathaniel Bacon in 1638. Bacon's Fearefull Estate of Francis Spira, based on the original Latin records, became an instant best-seller and was reissued ten times in England and eight times in the American colonies in the seventeenth and eighteenth centuries. For anti-Puritans, Spira's case exemplified the dangers of the Calvinist teachings of predestination and human corruption. Puritans, however, empathized with Spira, seeing Spira's condition as simply an extreme example of the experience of all godly Christians."
http://history.hanover.edu/courses/excerpts/260spira.html
This page contains an abbreviated version of Bacon's account of Spira, from which Watson presumably knew the story.

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Note added at 1 hr (2012-10-10 07:52:25 GMT)
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"Spira" became the archetype of the apostate, as in the title of this work of 1684:

"The English Spira: being a fearful example of an apostate who had been a preacher many years and then apostatized from his religion, miserably hanged himself, October the 13th, 1684 : giving an account of his dispair, and divers conferences had with him, by several ministers and others of his friends : together with his answer, and papers written by his own hand / left attested by Mr. T. Plant, Mr. H. Collings, Mr. B. Dennis, Mr. B. Keach"
http://catalogue.nla.gov.au/Record/5921139

His real name was presumably Francesco, but he was known as Francis in English.

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Note added at 1 hr (2012-10-10 08:01:14 GMT)
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Another interesting reference that demonstrates how celebrated Spira's case was soon after his death in 1550:

Giorgio Siculo, Epistola ... alli cittadini di Riva di Trento contra il mendatio di F. Spira, & falsa dottrina di Protestanti (Bologna, 1550)
http://www.swissbib.ch/TouchPoint/perma.do?v=nose&l=de&q=0="...[3]