The short pontificate of this Roman bishop, which lasted from the 18th of March, A. D. 417, until his death on the 26th of December in the following year, was rendered more remarkable by the rash activity with which he plunged into delicate and irritating controversies than by any display of sound judgment or high principle. His attention was first occupied by the representations of Caelestius and Pelagius, who, having appealed to his predecessor Innocentius against what they termed the harsh and prejudiced sentence of the Carthaginian synod, now earnestly demanded a full investigation of the charges preferred against their orthodoxy. Zosimus not only pronounced the complete acquittal of the accused, but inveighed in the strongest terms against the conduct of the African clergy, and published a letter testifying his entire satisfaction with the explanations of Pelagius. But scarcely had he given expression to these feelings when a total change was wrought in his sentiments by the edict of Honorius, issued at Ravenna on the last day of April, A. D. 418. Not satisfied with retracting the praise lavished on the two friends, he hastened to denounce them both as incorrigible heretics. and despatched a circular epistle (Tractoria) to convey a formal announcement of this condemnation to all the ecclesiastical authorities in the Christian world.

His next encounter was with Proculus of Marseilles, whom, along with Hilarius of Narbonne, and Simplicius of Vienne. he desired to make subordinate to the see of Arles, at that time occupied by a certain Patroclus, a priest of very doubtful reputation. The bishops of Narbonne and Vienne gave way to a certain extent, or at least did not peremptorily refuse obedience, but Proculus, warmly supported by his clergy and people, bade open defiance to his commands and excommunications.

Nothing discouraged by this repulse, Zosimus, within a very short period of his death, boldly asserted his absolute jurisdiction over the African church by reinstating a certain Apiarius. a presbyter of Sicca, who had been regularly deposed for various grave offences by his own diocesan, thus excititing a storm among the fiery Numidians, which must have produced a violent convulsion had the author of the decree lived to follow up this stretch of power by ulterior measures.