 Calvin also wrote many letters and treatises . Following the Responsio ad Sadoletum , Calvin wrote an open letter at the request of Bucer to Charles V in 1543 , Supplex exhortatio ad Caesarem , defending the reformed faith . This was followed by an open letter to the pope ( Admonitio paterna Pauli III ) in 1544 , in which Calvin admonished Paul III for depriving the reformers of any prospect of rapprochement . The pope proceeded to open the Council of Trent , which resulted in decrees against the reformers . Calvin refuted the decrees by producing the Acta synodi Tridentinae cum Antidoto in 1547 . When Charles tried to find a compromise solution with the Augsburg Interim , Bucer and Bullinger urged Calvin to respond . He wrote the treatise , Vera Christianae pacificationis et Ecclesiae reformandae ratio in 1549 , in which he described the doctrines that should be upheld , including justification by faith . 
