Career
It was Pope Felix III who was responsible for the solemn declaration of the excommunication and deposition of Acacius, bishop of Constantinople, on the ground of his authorship of the Henoticon, a doctrinal statement intended to supersede the Definition of the Council of Chalcedon. That council had accepted and acclaimed Pope Leo I's doctrine concerning Christ: two natures, divine and human, in one person. This doctrine was rejected by the Monophysites, who maintained that Christ had but one nature, the divine. The Henoticon was thus written to conciliate the Monophysites. The schism between the sees of Rome and Constantinople (the Acacian schism) lasted from 484 to 519.
Of Gelasius' writings some sixty letters and six theological treatises survive. These show him as an energetic and consistent upholder of papal primacy against Constantinople over the Council of Chalcedon. In Epistle 12, addressed to the Emperor Anastasius I, he clearly defined the relationship of the sacerdotium (the priestly power) and the imperium (the imperial power), exercising thereby a profound influence on the ideas of later centuries. Both types of authority are declared to be of divine origin, and within their respective spheres are independent and possess equivalent rights.
His assertion of papal primacy took the form of insistence on the principle that the decisions of episcopal synods are dependent for their validity on papal confirmation. In his capacity as bishop of Rome and metropolitan of southern Italy, he vigorously resisted a revival of Pelagianism, the heresy that denied the doctrine of original sin; gave warning of the insidiousness of the dualist heresy of Manichaeism; prohibited the observance of the pagan festival of Lupercalia (February 15); and eagerly promoted the enforcement of ecclesiastical discipline. Many of his disciplinary enactments were subsequently embodied in collections of Western canon law.
At the time of Gelasius' death the Acacian schism was unresolved, and it was left to a more peaceable and generous successor, Pope Hormisdas, eventually to achieve a solution in 519.