This is an exciting testimony us here Zweig, with literary genius we know. As noted in another comment above, I am not able to establish the historical reliability of the assertions Zweig, although I did qqs further research, especially on Wikipedia. Moreover, as in other biographies of Zweig (Erasmus, in particular), will certainly meet here elements linked to the dramatic time (30 years, Austria), during which Zweig wrote this.
But this does not alter the substance of the text, that is to say to the author's thoughts about tolerance, about fanaticism, about totalitarianism which Calvin was apparently one of the first major modern figures.
Castellion Why is it not better known in Geneva and in Switzerland?
Some of these words are a remarkable form, like "Killing a man is not to defend a doctrine is to kill a man. When the Genevans were destroyed Servetus they did not defend a doctrine, they killed a human being it does not prove his faith by burning a man but by being burned for her. "
In short, regardless of historical accuracy, I can not estimate, the book remains a strong work, rich, which gives much food for thought and to better understand both the past and the present.