Sergius I of Constantinople (Greek: Σέργιος, Sergios; died 9 December 638) was the Ecumenical Patriarch of Constantinople from 610 to 638. He is most famous for promoting Monothelitism Christianity, especially through the Ecthesis.
Sergius I was born of Syrian Jacobite heritage. He first came to power as Ecumenical Patriarch of Constantinople in 610. He was also a known supporter of Emperor Heraclius, crowning Heraclius as emperor himself in 610. Sergius I also provided support to Heraclius throughout his campaign against the Persians. Sergius I also played a prominent role in the defense of Constantinople against the combined Avar-Persian-Slavic forces during their invasion of Constantinople in 626. Sergius I's connections to both political and religious authorities gave him to his influence in both the religious and political communities to further Monoenergism as the primary formula of Christ within the church. This was met with much opposition, especially from that of the Chalcedonian supporters, Maximus the Confessor and Sophronius of Jerusalem. In response to their resistance to accept the ideas of Monoenergism, Sergius I responded with the Ecthesis, a formula which forbade discussing the idea that the Person of Christ had one or two energies in favour of Monothelitism being the idea that the Person of Christ had two natures that were united by a single will. The Ecthesis was signed by Heraclius in 638, the same year that Sergius I died.
