 In 1556 , a Dutch dissident, held a public disputation with Calvin during his visit to Frankfurt, in which Velsius defended free will against Calvin's doctrine of predestination.Following the execution of Servetus, a close associate of Calvin, Sebastian Castellio, broke with him on the issue of the treatment of heretics.In Castellio's "Treatise on Heretics" (1554), he argued for a focus on Christ's moral teachings in place of the vanity of theology, and he afterward developed a theory of tolerance based on biblical principles.