 Calvin also wrote many letters and treatises . Following the <unk> ad <unk> , Calvin wrote an open letter at the request of Bucer to Charles V in 1543 , <unk> <unk> ad Caesarem , defending the reformed faith . This was followed by an open letter to the pope ( <unk> <unk> 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 <unk> <unk> cum <unk> 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 <unk> et Ecclesiae <unk> ratio in 1549 , in which he described the doctrines that should be upheld , including justification by faith . 
