Pope Nicholas I, (Rome c. 820 – November 13, 867), or Nicholas the Great, reigned from April 24, 858 until his death. He is remembered as a consolidator of papal authority and power, exerting decisive influence upon the historical development of the papacy and its position among the Christian nations of Western Europe, and is considered a saint.
He refused to grant an annulment to Lothar II from Theutberga so that Lothar could marry his mistress Waldrada; when a Council pronounced in favor of annulment, Nicholas I declared the Council to be deposed, its messengers excommunicated, and its decisions void. Despite pressure from the Carolingians, who laid siege to Rome, his decision held. During his reign, relations with the Byzantine Empire soured over his support for Ignatius as Patriarch of Constantinople, who had been removed and Photius appointed to replace him.
Going in person to Ravenna, the pope then investigated and equitably regulated everything. Again appealing to the emperor, the archbishop was recommended by him to submit to the pope, which he did at the Roman Synod of November, 861. Later on, however, he entered into a pact with the excommunicated archbishops of Trier and Cologne, was himself again excommunicated, and once more forced to make his submission to the pope. Another conflict arose between Nicholas and Archbishop Hincmar of Reims: this concerned the prerogatives of the papacy. Bishop Rothad of Soissons had appealed to the pope against the decision of the Synod of Soissons, of 861, which had deposed him; Hincmar opposed the appeal to the pope, but eventually had to acknowledge the right of the papacy to take cognizance of important legal causes (causae majores) and pass independent judgment upon them. A further dispute broke out between Hincmar and the pope as to the elevation of the cleric Wulfad to the archiepiscopal See of Bourges, but here, again, Hincmar finally submitted to the decrees of the Apostolic See, and the Frankish synods passed corresponding ordinances.
The pope was also involved in a desperate struggle with Lothair II of Lorraine over the inviolability of marriage. Lothair had abandoned his lawful wife Theutberga to marry Waldrada. At the Synod of Aachen, 28 April 862, the bishops of Lorraine approved this union, contrary to ecclesiastical law. At the Synod of Metz, June, 863, the papal legates, bribed by the king, assented to the Aachen decision, and condemned the absent Theutberga. Upon this the pope brought the matter before his own tribunal. The two archbishops, Günther of Cologne and Thietgaud of Trier, who had come to Rome as delegates, were summoned before the Lateran Synod of October, 863, when the pope condemned and deposed them as well as John of Ravenna and Hagano of Bergamo. The Emperor Louis II took up the cause of the deposed bishops, while King Lothair advanced upon Rome with an army and laid siege to the city, so that the pope was confined for two days in St. Peter's without food. Yet Nicholas did not waver in his determination; after being reconciled with the pope, the emperor withdrew from Rome and commanded the former Archbishops of Trier and Cologne to return to their homes. Nicholas never ceased from his efforts to bring about a reconciliation between Lothair and his lawful wife, but without effect.
Another matrimonial case in which Nicholas interposed was that of Judith, daughter of Charles the Bald, who had married Baldwin, Count of Flanders, without her father's consent. Frankish bishops had excommunicated Judith, and Hincmar of Reims had taken sides against her, but Nicholas urged leniency, in order to protect freedom of marriage.
Photius was elected lawfully and canonically in 858 according to the Eastern church. This was affirmed two months later in a true ecumenical council where Ignatios’ elevation to the Patriarchate was declared to be uncanonical and Photius was acclaimed as properly elected as the new Patriarch. This led to conflict between East and West over doctrinal issues such as the Filioque and territorial claims due to Nicholas stance as a “ruler” over the church. An ecumenical synod was convened in 867 in Constantinople where Nicholas was excommunicated along with the rejection of his claims of primacy, his intrusion into Bulgaria, and the innovative addition of the Filioque.
Bulgaria having been converted by Greek missionaries, its ruler, Prince Boris, in August, 863, sent an embassy to the pope with one hundred six questions on the teaching and discipline of the Church. Nicholas answered these inquiries exhaustively in his "Responsa Nicolai ad consulta Bulgarorum" (Mansi, "Coll. Conc.", XV, 401 sqq.). At the same time he sent an embassy to Prince Boris in an abortive attempt to convert him to the western usage; ultimately the prince elected to join the Eastern Church.
At Rome, Nicholas rebuilt and endowed several churches, and constantly sought to encourage religious life. His led a pious personal life guided by a spirit of Christian asceticism. Regino of Prüm reports that Nicholas was highly esteemed by the citizens of Rome and by his contemporaries generally (Chronicon, "ad annum 868," in "Mon. Germ. Hist." Script.", I.579), and after death was regarded as a saint.
A much discussed question and one that is important in judging the position taken by this pope is whether he made use of the forged pseudo-Isidorian papal decretals. After exhaustive investigation, Schrörs has decided that the pope was neither acquainted with the pseudo-Isidorian collection in its entire extent, nor did he make use of its individual parts; that he had perhaps a general knowledge of the false decretals, but did not base his view of the law upon them, and that he owed his knowledge of them solely to documents which came to him from the Frankish Empire [Schrörs, "Papst Nikolaus I. und Pseudo-Isidor" in Historisches Jahrbuch, XXV (1904), 1 sqq.; Idem, "Die pseudoisidorische 'Exceptio spolii' bei Papst Nikolaus I" in Historisches Jahrbuch, XXVI (1905), 275 sqq.].