The Agnus Dei and the Cross Restored

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.