Nicholas the Great was elected pope in 858 at thirty-eight, in the presence of the Frankish king Louis II — the only pope between Gregory the Great and Innocent III to receive the title 'the Great' from later Roman tradition. His nine-year pontificate decisively shaped the medieval papacy.
He compelled the Lotharingian King Lothair II to take back the wife he had unjustly divorced, defending the indissolubility of marriage even against Frankish royal pressure. He deposed and rebuked the archbishops of Trier and Cologne who had collaborated in the divorce. He directly intervened in the Bulgarian and Slavic missions, sending the Responsa ad Bulgaros (866) — one of the most extensive papal documents on the Christianization of a pagan people.
He confronted the Byzantine Patriarch Photius over the irregular replacement of Patriarch Ignatius, in the controversy known as the Photian Schism — the first major break in Latin-Greek ecclesiastical relations after the iconoclast period. Nicholas's repeated rulings against Photius defined the Roman position on papal jurisdiction in the universal Church for centuries.
He restored the See of Reims and ruled definitively against Archbishop Hincmar on multiple disputes — establishing the precedent that the local archbishop, however powerful, must yield to Rome on disputed canonical matters. He died in Rome in 867 having decisively raised the practical authority of the papacy in the Latin Church.