Book III. Augustin goes on to refute other matters which are calumniously objected by the Pelagians in the same letter sent to Thessalonica; and expounds, in opposition to their heresy, what those who are truly Catholic say concerning the utility of the law; what they teach of the effect and virtue of baptism; what of the discrepancy between the two Testaments, the Old and the New; what concerning the righteousness and perfection of the prophets and apostles; what of the appellation of sin in Christ, when He is said in the likeness of sinful flesh concerning sin to have condemned sin, or to have become sin; and finally, what they profess concerning the fulfilment of the commandments in the future life.