Photios I of Constantinople (b. 810)
Photios I (Greek: Φώτιος, Phōtios; c. 810/820 – 6 February 893), also spelled Photius (), was the ecumenical patriarch of Constantinople from 858 to 867 and from 877 to 886. He is recognized in the Eastern Orthodox Church as Saint Photios the Great.

Photios is widely regarded as the most powerful and influential church leader of Constantinople subsequent to John Chrysostom's archbishopric around the turn of the fifth century. He is also viewed as the most important intellectual of his time – "the leading light of the ninth-century renaissance". He was a central figure in both the conversion of the Slavs to Christianity and the Photian schism, and is considered "[t]he great systematic compiler of the Eastern Church, who occupies a similar position to that of Gratian in the West," and whose "collection in two parts...formed and still forms the classic source of ancient Church Law for the Greek Church."Photios was a well-educated man from a noble Constantinopolitan family. Photios's great uncle was a previous patriarch of Constantinople, Saint Tarasius. He intended to be a monk, but chose to be a scholar and statesman instead. In 858, Emperor Michael III (r. 842–867) decided to confine Patriarch Ignatius in order to force him into resignation, and Photios, still a layman, was appointed to replace him. Amid power struggles between the pope and the Byzantine emperor, Ignatius was reinstated. Photios resumed the position when Ignatius died (877), by order of the Byzantine emperor. The new pope, John VIII, approved Photios's reinstatement. Catholics regard as legitimate a Fourth Council of Constantinople (Roman Catholic) anathematizing Photios, while Eastern Orthodox regard as legitimate a subsequent Fourth Council of Constantinople (Eastern Orthodox), reversing the former. The contested councils mark the end of unity represented by the first seven Ecumenical Councils.

Studies show that Photios was venerated as a saint as early as the 9th century, and by the Roman Church as late as the 12th century. Nonetheless, Photios was formally canonized by the Eastern Orthodox Church in 1847.