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 (s. Agostino, Ep. 193: PL 38, 869); in 418 he sent from Rome to s. Agostino letters against the Pelagians, which have nevertheless 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 knew. M. M. had no original ideas, depending closely on Cirillo di Alessandria and Agostino; his polemic aimed above all at setting his adversaries against one another, but only very rarely addressed the heart of the matter. He was greatly concerned by the disputes disturbing the Churches of East and West, which explains his effort to bring into contact two such different heresies as Nestorianism and Pelagianism.
The works of M. M., preserved in a single manuscript (Palat. lat. 234 of the Bibl. Vaticana), 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 the Opus imperfectum contra Iulianum of s. Agostino [cf. E. Schwartz, op. cit., pp. 3–14]); Refutatio symboli Nopsuestani (ibid., pp. 15–18); booklets against Nestorius and versions of letters by Nestorius and Cirillo Alessandrino (ibid., pp. 19–29; cf. p. XII and F. Loofs, Nestoriana, Halle 1905, pp. 103, 134–46); excerpts from 4 anti-Pelagian sermons by Nestorius (E. Schwartz, op. cit., pp. 34–35); letter of Nestorius and Coelestius (ibid., p. 35; cf. F. Loofs, op. cit., pp. 146–50); Commonitorium super nomine Coelestii (ibid., p. 36: this 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 the Corpus of M. M.’s works comes to an end; the older editions wrongly attribute the following versions to him: Cyrilli et Nestorii XII capitula (E. Schwartz, op. cit., p. 37); Gesta Ephesiana (protocol of the session of 22 July: ibid., p. 38); defence of Cirillo against the Antiochene party (ibid., p. 39); defence of Cirillo against Theodoret (ibid., p. 39); excerpts from Theodoret, Theodore of Nopsuestia, Diodore of Tarsus, Ibas of Edessa, and Eutherius of Thiana (ibid., pp. 41–54); later additions (ibid., pp. 55–57); cf. Bardenhewer, IV, p. 529).