MARIO MERCATORE. - Latin writer of the 5th century. According to E. Schwartz (Acta Conc. Oec. I, 5, 1 [1924], pp. 3-70), who identified his authentic works and determined what can be known of his life, M. M. was African (St. Augustine, Ep. 193: PL 38, 869); in 418 he sent from Rome to St. Augustine some letters against the Pelagians, which however have been lost; in 429 he was in Constantinople, where he devoted himself to translations and published treatises against the Pelagians and the Nestorians; all trace of him is lost after the Council of Ephesus, which he certainly attended. M. M. had no original ideas, depending strictly on Cyril of Alexandria and on Augustine; his polemic aimed above all at setting his adversaries in opposition to one another, but only very rarely did he confront the heart of the question. He appears greatly concerned by the disputes that disturbed the Churches of East and West, which explains his effort to bring into contact two heresies so different from each other as Nestorianism and Pelagianism.
The works of M. M., preserved in a single manuscript (Palat. lat. 234 of the Vatican Library), are: Marii Mercatoris servi Christi commontorium lectori adversus haeresim Pelagii et Coelestii vel etiam scripta Iuliani (this writing is later than 430, because it makes use of St. Augustine's Opus imperfectum contra Iulianum [cf. E. Schwartz, op. cit., pp. 3-14]); Refutatio symboli Nopsuestani (ibid., pp. 15-18); pamphlets against Nestorius and a version of letters of Nestorius and of Cyril of Alexandria (ibid., pp. 19-29; cf. p. XII and F. Loofs, Nestoriana, Halle 1905, pp. 103, 134-46); extracts of 4 anti-Pelagian sermons of Nestorius (E. Schwartz, op. cit., pp. 34-35); a letter of Nestorius and Coelestius (ibid., p. 35; cf. F. Loofs, op. cit., pp. 146-50); Commonitorium super nomine Coelestii (ibid., p. 36: it is a treatise, addressed by M. M. in Greek to the Church of Constantinople and in Latin to Theodosius II; on the sources of this writing cf. E. Schwartz, op. cit., p. x).
Here ends the Corpus of the works of M. M.; the old editions wrongly attribute to him the following versions: Cyrilli et Nestorii XII capitula (E. Schwartz, op. cit., p. 37); Gesta Ephesiana (protocol of the session of 22 July: ibid., p. 38); defense of Cyril against the Antiochene party (ibid., p. 39); defense of Cyril against Theodoret (ibid., p. 39); extracts of Theodoret, of Theodore of Nopsuestia, of Diodore of Tarsus, of Ibas of Edessa and of Eutherius of Tyana (ibid., p. 41-54); later additions (ibid., pp. 55-57); cf. Bardenhewer, IV, p. 529).