FL. c.418-452. Latin Christian polemicist. Augustine's* grateful response to two anti-Pelagian writings (now lost) submitted to him by Marius places this layman in Rome about 418. Probably originally from Africa, he resided in Constantinople from about 429 to about 451, perhaps as Pope Celestine's legate. His treatise of 429 against Celestius, presented to , and his rebuttal of * helped secure the imperial banishment of these Pelagians from Constantinople. His works on the nature of Christ against * and Nestorianism,* supporting the position of Cyril* of Alexandria, aided in the condemnation of both heresies at the * in 431. Marius's extant writing consists chiefly of Latin translations of heretical treatises and orthodox refutations, rendering him a major source for the history of the Nestorian and Pelagian controversies.