 In March 1536 , Calvin published the first edition of his <unk> Christianae <unk> or Institutes of the Christian Religion . The work was an apologia or defense of his faith and a statement of the doctrinal position of the reformers . He also intended it to serve as an elementary instruction book for anyone interested in the Christian religion . The book was the first expression of his theology . Calvin updated the work and published new editions throughout his life . Shortly after its publication , he left Basel for Ferrara , Italy , where he briefly served as secretary to Princess Renée of France . By June he was back in Paris with his brother Antoine , who was resolving their father 's affairs . Following the Edict of Coucy , which gave a limited six @-@ month period for heretics to reconcile with the Catholic faith , Calvin decided that there was no future for him in France . In August he set off for Strasbourg , a free imperial city of the Holy Roman Empire and a refuge for reformers . Due to military manoeuvres of imperial and French forces , he was forced to make a detour to the south , bringing him to Geneva . 
