1542-1553. Working as a physician in Vienne

During his twelve-year residence in Vienne, the longest quiet period of his troubled life, Servetus acquired fame and fortune as a physician and, at the same time, he continued working as proof corrector. In 1542, he brought out a new edition of Ptolemy which he softened down some of the notes that had given offense before. He next prepared an edition of the Santes Pagnini's Bible, completed in seven volumes in 1545. His introduction and notes anticipate modern biblical criticism and show a marked advance in sophistication beyond that of his earlier theological writing.

At the same time, Michel de Villeneuve, Servetus continued cultivating his interest in theology with the prepararion of his major theological treatise, Christianismi Restitutio (The Restoration of Christianity). He also began, in 1546, a fateful secret correspondence with his old acquaintance, John Calvin. By this time Calvin, author of Institutio Christianae Religionis (Institutions of Christian Religion), 1536, and pastor and chief reformer of Geneva, was the most prestigious figure in the Reform branch of Protestantism.

Calvin's theology had included little mention of the trinitarian nature of the godhead until, in 1537, another reformer, Pierre Caroli, accused him of being an Arian. Although cleared by a synod at Lausanne, Calvin was afterwards on his guard and determined to deal severely with deviations in this area of orthodoxy. The subject, associated with unpleasant memories, was distasteful to him. Servetus, surely aware of Calvin's previous lack of clarity on the subject, bombarded him with letters insisting on unorthodox conceptions more radical than those he had presented a decade and more ago. Calvin replied with increasing impatience and asperity. Servetus sent Calvin a manuscript of his yet unpublished Restitutio. Calvin reciprocated by sending a copy of the Institutio. Servetus returned it with abusive annotations. On the day Calvin broke off the correspondence, he wrote to his colleague, Guillaume Farel, that should Servetus ever come to Geneva, "if my authority is of any avail I will not suffer him to get out alive."