Year of birth unknown; died 27 December 418. After the death of Pope Innocent I on 12 March 417, Zosimus was elected his successor.
According to the “Liber Pontificalis” Zosimus was a Greek and his father’s name was Abram. Harnack wished to deduce from this name that the family was of Jewish origin, but the statements of the “Liber Pontificalis” in respect to the families of the popes of this period cannot always be regarded as exact.
Nothing is known of the life of Zosimus before his elevation to the papal see. His consecration as Bishop of Rome took place on 18 March 417. The festival was attended by Patroclus, Bishop of Arles, who had been raised to that see in place of Bishop Hero, who had been forcibly and unjustly removed by the imperial general Constantine. Patroclus gained the confidence of the new pope at once; as early as 22 March he received a papal letter which conferred upon him the rights of a metropolitan over all the bishops of the Gallic provinces of Viennensis and Narbonensis I and II. In addition he was made a kind of papal vicar for the whole of Gaul, no Gallic ecclesiastic being permitted to journey to Rome without bringing with him a certificate of identity from Patroclus.
In the year 400 Arles had been substituted for Trier as the residence of the chief government official of the civil Diocese of Gaul, the “Prefectus Praetorio Galliarum”. Patroclus, who enjoyed the support of the commander Constantine, used this opportunity to procure for himself the position of supremacy above mentioned, by winning over Zosimus to his ideas. The bishops of Vienne, Narbonne, and Marseilles regarded this elevation of the See of Arles as an infringement of their rights, and raised objections which occasioned several letters from Zosimus. The dispute, however, was not settled until the pontificate of Pope Leo I. Not long after the election of Zosimus the Pelagian Coelestius, who had been condemned by the preceding pope, Innocent I, came to Rome to justify himself before the new pope, having been expelled from Constantinople. In the summer of 417 Zosimus held a meeting of the Roman clergy in the Basilica of Saint Clement before which Coelestius appeared. The propositions drawn up by the Deacon Paulinus of Milan, on account of which Coelestius had been condemned at Carthage in 411, were laid before him. Coelestius refused to condemn these propositions, at the same time declaring in general that he accepted the doctrine expounded in the letters of Pope Innocent and making a confession of faith which was approved. The pope was won over by the shrewdly calculated conduct of Coelestius, and said that it was not certain whether the heretic had really maintained the false doctrine rejected by Innocent, and that therefore he considered the action of the African bishops against Coelestius too hasty. He wrote at once in this sense to the bishops of the African province, and called upon those who had anything to bring against Coelestius to appear at Rome within two months. Soon after this Zosimus received from Pelagius also an artfully expressed confession of faith, together with a new treatise by the heretic on free will. The pope held a new synod of the Roman clergy, before which both these writings were read. The skilfully chosen expressions of Pelagius concealed the heretical contents; the assembly held the statements to be orthodox, and Zosimus again wrote to the African bishops defending Pelagius and reproving his accusers, among whom were the Gallic bishops Hero and Lazarus. Archbishop Aurelius of Carthage quickly called a synod, which sent a letter to Zosimus in which it was proved that the pope had been deceived by the heretics. In his answer Zosimus declared that he had settled nothing definitely, and wished to settle nothing without consulting the African bishops. After the new synodal letter of the African council of 1 May, 418, to the pope, and after the steps taken by the Emperor Honorius against the Pelagians, Zosimus recognized the true character of the heretics. He now issued his “Tractoria”, in which Pelagianism and its authors were condemned. Thus, finally, the occupant of the Apostolic See at the right moment maintained with all authority the traditional dogma of the Church, and protected the truth of the Church against error.
Shortly after this Zosimus became involved in a dispute with the African bishops in regard to the right of appeal to the Roman See clerics who had been condemned by their bishops. When the priest Apiarius of Sicca had been excommunicated by his bishop on account of his crimes he appealed directly to the pope, without regard to the regular course of appeal in Africa which was exactly prescribed. The pope at once accepted the appeal, and sent legates with letters to Africa to investigate the matter. A wiser course would have been to have first referred Apiarius to the ordinary course of appeal in Africa itself. Zosimus next made the further mistake of basing his action on a reputed canon of the Council of Nicaea, which was in reality a canon of the Council of Sardica. In the Roman manuscripts the canons of Sardica followed those of Nicaea immediately, without an independent title, while the African manuscripts contained only the genuine canons of Nicaea, so that the canon appealed to by Zosimus was not contained in the African copies of the Nicene canons. Thus a serious disagreement arose over this appeal, which continued after the death of Zosimus.
Besides the writings of the pope already mentioned, there are extant other letters to the bishops of the Byzantine province in Africa, in regard to a deposed bishop, and to the bishops of Gaul and Spain in respect to Priscillianism and ordination to the different grades of the clergy. The “Liber Pontificalis” attributes to Zosimus a Decree on the wearing of the maniple by deacons and on the dedication of Easter candles in the country parishes; also a Decree forbidding clerics to visit taverns. Zosimus was buried in the sepulchral Church of Saint Laurence in Agro Verano.