According to the Liber pontificalis, Zosimus was of Greek origin whose family had settled on the east coast of the Italian peninsula in Mesoraca, Calabria. As is typical of this time, nothing is known about the man until he rose to the Chair of Peter.
Zosimus was consecrated 18 March 417. Within four days, he was already making waves in the hierarchy. Previously, in 400, Arles, in Gaul, had replaced Trier as the residence of the chief government official of the civil diocese of Gaul. The Roman general Constantine (not to be confused with the emperor of the same name who had died several generations before) removed Heros as bishop of Arles. In his place was put a favorite of Constantine's, Patroclus. This bishop claimed supremacy over the other bishops of the region, which included Vienne, Narbonne and Marseille. Patroclus attended the celebrations attendant upon the new pope's rise to the throne and presented his relatively new position within days. Apparently, he also had a reason why he was in a supreme position. Using the apocraphyl story of the Council of Nicea having ruled the rights of metropolitan bishop to all bishops of Arles, Patroclus essentially became the pope's agent to Gaul. Zosimus promptly wrote a letter to Patroclus agreeing with this stance. With this came not only the supremacy over a region, but also the demand for a letter of introduction from Patroclus to accompany any cleric who wished to see the pope. Needless to say, the other bishops in the region were less than accepting of this arrangement. So, Zosimus wrote to them demanding that they accept the circumstances or be excommunicated. The slow-burning argument remained for several more decades until Pope Leo settled it.
No sooner had Zosimus declared himself on the Arles problem, Celestius, a Pelagian teacher, showed himself in Rome. Celestius, a Catholic deacon, had been exiled from Constantinople by the bishop there. His teachings were considered heretical. Celestius had written to Pope Innocent, asking for a recourse, but the letter arrived after the death of that pope, so he chose to plead his case to the new one. Celestius was a smooth talker and a good writer. His presentation seemed perfectly orthodox to Zosimus and the pope defended the man. Likewise, when Pelagius came to Rome and defended himself, by written and spoken testimony, the pope supported him, also. He wrote to the African bishops as to his views. It was not until months later that the two men, Celestius and Pelagius, seemed to forget what they had testified and began preaching in North Africa. At this point, the bishops called a synod to discuss the matter. From the synod came a letter to the pope explaining that Pelagius and his teachings were heretical. This was followed up by the western emperor, Honorius, declaring Pelagianism a threat and condemning it. This is when Zosimus truly changed his mind and declared the teaching heretical. His work, Tractoria, is the condemnation of this teaching.
Zosimus was involved in a number of jurisdictional problems, mostly involving the African churches. Not always following the regular course of appeal came from a mistaken reading of council records. This led to a lengthy argument which lasted past Zosimus' reign. He also wrote to the churches of Gaul and Spain warning them of Priscillianism, another heresy.
Zosimus had some discipline issues which he addressed, including proper vestments and forbidding clerics to enter taverns, probably with some justification.
This pope has been harangued since his death over his change of opinion on the heresy of Pelegius and Celestius. The most prevalent current defense is that papal infallibility does not cover everything a pope says, but only those things that address faith and doctrine, said ex cathedra. Claiming a person a heretic was not addressing faith and doctrine per se, nor was it claimed ex cathedra. Thus, he was not expected to be speaking infallibly.
Pope Zosimus died December 26, 418, after 21 months of rule. He is buried in the Basilica of Saint Lawrence Outside the Walls. His time as pope is seen as a time of dissension and shows that even the Rock of St. Peter can sometimes be unstable.
Needless to say, Pope Zosimus did not reign long enough to repair the broken relationships. The Roman clergy was quite split on procedures and discipline after his death. Dual elections were held and two popes temporarily took over. Eventually the two popes were exiled and troubles led to the Roman soldiers taking over control of the city.