Pope Gelasius I was the bishop of Rome from 1 March 492 to his death on 21 November 496. Gelasius was a prolific author whose style placed him on the cusp between Late Antiquity and the Early Middle Ages. Some scholars have argued that his predecessor Felix III may have employed him to draft papal documents, although this is not certain.

During his pontificate he called for strict Catholic orthodoxy, more assertively demanded obedience to papal authority, and, consequently, increased the tension between the Western and Eastern Churches. Surprisingly, he also had cordial relations with the Ostrogoths, who were Arians (i.e., Non-trinitarian Christians), and therefore perceived as heretics from the perspective of Nicene Christians.

There is some confusion regarding where Gelasius was born: according to the Liber Pontificalis he was born in the Roman province of Africa (present-day Tunisia), referred to as "natione Afer", while in a letter addressed to the Roman Emperor Anastasius he stated that he was "born a Roman" ("Romanus natus"). J. Conant opined that the latter assertion probably merely denotes that he was born in Roman Africa before the Vandals invaded it.

The papal election of Gelasius on 1 March 492 was a gesture of continuity: Gelasius inherited the conflicts of Pope Felix III with Eastern Roman Emperor Anastasius and the patriarch of Constantinople and exacerbated them by insisting on the obliteration of the name of the deceased Patriarch Acacius of Constantinople from the diptychs, in spite of every ecumenical gesture offered by the contemporaneous Patriarch Euphemius.

The split with the Emperor and the Patriarch of Constantinople was inevitable, from the Western view, because they considered the Monophysite view of Jesus Christ having only a Divine nature a heresy. Gelasius authored the book De duabus in Christo naturis (On the dual nature of Christ), which described Catholic doctrine in the matter. Thus Gelasius, for all the conservative Latinity of his style of writing, was on the cusp of Late Antiquity and the Early Middle Ages.

During the Acacian schism, Gelasius advocated the primacy of the See of Rome over the universal Church, both East and West, and he presented this doctrine in terms that became the model for successive popes, who also claimed papal supremacy because of their succession to the papacy from the first supreme pontiff, Peter the Apostle.

In 494, Gelasius authored the very influential letter Duo sunt to Anastasius on the subject of the relation of Church and state, which letter had political impact for more than a millennium: Pope Gregory XVI quoted from it in his letter to the Swiss clergy, Commissum divinitus (17 May 1835), responding to the Baden articles [de], which gave some of the Swiss cantons authority over church matters including the sacraments.

Closer to home, after a long contest Gelasius finally suppressed the ancient Roman festival of the Lupercalia, which had persisted for several generations among a nominally Christian population. Gelasius' letter to the senator Andromachus treated the primary contentions of the controversy and incidentally provided some details of the festival, which combined fertility and purification, that might have been lost otherwise.