Sergius I

In 692 the Byzantine emperor Justinian II convened a council in the domed hall of the imperial palace known as the Trullo, meant to complete the work of the fifth and sixth ecumenical councils by issuing disciplinary canons neither had addressed, which is why it is called the Quinisext, fifth-sixth, Council. Its 102 canons touched on clerical marriage, fasting practices, and much else, and several directly criticized or overrode Western practices, including Roman fasting customs and priestly celibacy, treating Constantinople's usage as the standard the whole church should follow.

Sergius, presented with the canons for his signature as bishop of Rome, refused outright, reportedly declaring he would rather die than consent to erroneous novelties, and would not even permit the document to be read aloud in his presence. Justinian II responded as an emperor accustomed to obedience from the church within his reach: he had two of the pope's representatives in Constantinople arrested, and dispatched Zacharias, the commander of his personal bodyguard, to Rome with orders to seize Sergius and bring him back for trial, much as an earlier emperor had once had Pope Martin I dragged from Rome to exile.

This time the plan failed. The Roman citizen militia and troops from the exarchate of Ravenna rallied around the pope, surrounded the imperial residence, and forced Zacharias to flee for his life, reportedly hiding under Sergius's own bed before the pope intervened to save him from the crowd. Justinian was deposed not long afterward in a palace revolution, and the confrontation stood as an early, concrete sign that Rome, though still nominally under Byzantine authority, would no longer simply submit to an emperor's religious dictates by force.

Alongside his confrontation with Constantinople, Sergius left a lasting mark on the Roman liturgy itself. He introduced the chant Agnus Dei, Lamb of God, into the Mass, sung while the bread is broken before communion, a text and gesture still in continuous use in the Roman rite more than thirteen centuries later. He is also credited with restoring greater prominence to the feast of the Exaltation of the Holy Cross in Rome, a commemoration tied to the recovery and veneration of relics of the true cross, at a moment when Rome's own connections to relics and processions from the eastern Mediterranean were becoming an increasingly important part of its religious identity.

His pontificate also shows the early medieval papacy acting as a diplomatic magnet for distant Christian rulers. Caedwalla, the pagan-born king of Wessex in Anglo-Saxon England who had converted to Christianity, traveled the long journey to Rome specifically to be baptized by the pope himself, arriving in 688 or 689. Sergius baptized him, giving him the name Peter, and Caedwalla died in Rome only days later, still wearing his baptismal robes, and was buried at St Peter's. The visit reflects how far Rome's religious authority already reached beyond the old Roman world by the late seventh century, drawing a king from the edge of Britain to the city for the single most important rite of his life.