Pope Gelasius I was elected after the death of Pope Felix III on March 1, 492 AD. At the time of his election, there was tension between the East and West, caused by the repudiation of the Henoticon by Felix. The Roman Emperor Zeno I issued the Henoticon in 482 to reconcile the differences between the Chalcedonian Christians and the Miaphysite Christians, by endorsing the condemnations of Eutyches and Nestorius at the Council of Chalcedon, and approving the twelve anathemas of Cyril of Alexandria, but avoiding the definition of whether Jesus Christ had one or two natures. With this, Zeno was trying to appease both sides, but instead failed to satisfy either. This is a category of papal coins.
In response, Felix had written two letters – one to Zeno and one to Acacius, the Patriarch of Constantinople – reminding them both of their duty to defend the faith without question. At the same time, John Talaia, Patriarch of Alexandria, refused to sign the Henoticon because of his adherence to the decisions of the Council of Chalcedon held in 451. Zeno expelled him and replaced him with a Miaphysite – Peter Mongus – who was recognized by the Patriarchs of Antioch and Constantinople. Talaia fled to Rome and told Felix what was happening in the East. Felix sent two more letters to Acacius, summoning him to Rome to explain the matter. When the legates arrived in Constantinople with the letters, they were imprisoned and forced to take Communion from Acacius during a Liturgy naming Peter Mongus in the diptychs. Felix received word of this and held a synod in 484 during which he denounced his legates and excommunicated Acacius. This led to the Acacian Schism whereby Acacius responded by removing Felix from the diptychs and imprisoning Cyril, the abbot of the Acoemeti Monks in Constantinople who were still loyal to Rome.
Acacian died in 489 and his successor, Phrabitas, tried to reconcile with Rome, but refused to strike Acacius from the diptychs or deny communion with the Miaphysites. Felix would not budge on his views and the Schism dragged on. Phrabitas died in 490, as did Peter Mongus. Zeno died in 491 and was succeeded by Anastasius. The East and West were still firmly opposed to the viewpoint of the other even through all of the changes of those in power. Anastasius was a Miaphysite and kept the policies of the Henoticon.
When Felix died the following year and Gelasius was named Pope, it was viewed as a continuation of the beliefs of Felix III. Gelasius worked closely with Felix, especially in the drafting of papal documents. He struck Acacius from the diptychs, called for a more strict orthodoxy and pushed for more papal authority.
In 494, Gelasius wrote the Duo sunt, to Anastasius, a letter outlining the separation of church and state, and the independent spheres of influence which were not to violate each others jurisdictions. This doctrine would be the underlying dualistic principle of Western European political thought for about the next millennium. On May 3, 495, Gelasius affirmed his authority over the entire Church, both East and West, when he presented his doctrine of papal supremacy through the succession of Roman Popes through the Apostle Peter.
Gelasius died in 496 and was succeeded by Anastasius II. Even though the name of the new Pope was Anastasius, he was unrelated to the current emperor and was also unable to end the Acacian Schism. The Schism would continue until the reign of Emperor Justin I when it would finally be reconciled and formalized on Easter, March 24, 519 AD.
Gelasius was the most prolific writer of the early Roman bishops. Between 37 and 42 letters, along with fragments of 49 others, are all archived at the Vatican.