He is venerated as a saint in the Roman Catholic Church, with a feast day on 13 November.
He refused to grant an annulment to King Lothair II of Lotharingia from Teutberga so that Lothair 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, Patriarch of Constantinople, who had been removed from his post in favor of Photius.
Born to a distinguished family in Rome, son of the Defensor Theodore, Nicholas received excellent training. Distinguished for his piety, benevolence, competence, knowledge, and eloquence, he entered the service of the Church at an early age. He was made a subdeacon by Pope Sergius II (844–847) and a deacon by Leo IV (847–855). After the death of Benedict III (7 April 858), Holy Roman Emperor Louis II came to Rome to exert his influence upon the election. On 24 April Nicholas was elected pope, consecrated, and enthroned in St. Peter's Basilica in the presence of the emperor.
Three days after, Nicholas held a farewell banquet for the emperor and afterward, accompanied by the Roman nobility, visited him in his camp before the city, on which occasion the emperor came to meet the pope and led his horse for some distance.
To a spiritually exhausted and politically uncertain Western Europe beset by Muslim and Norse incursions, Pope Nicholas appeared as a conscientious representative of Roman primacy in the Church. He was filled with a high conception of his mission for the vindication of Christian morality and the defence of God's law.
His co-operation with Emperor Louis II and Byzantine forces temporarily stemmed the Muslim advance in southern Italy. He also strengthened the Ostian fortifications against any future Muslim raids.
Archbishop John of Ravenna oppressed the inhabitants of the papal territory, treated his suffragan bishops with violence, made unjust demands upon them for money, and illegally imprisoned priests. He also forged documents to support his claims against the Roman See and maltreated the papal legates. As the warnings of the pope were without result, and the archbishop ignored a thrice-repeated summons to appear before the papal tribunal, he was excommunicated. Having first visited the Emperor Louis at Pavia, the archbishop repaired with two imperial delegates to Rome, where Nicholas cited him before the Roman synod assembled in the autumn of 860. Upon this John fled from Rome.
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.
Nicholas showed the same zeal in other efforts to maintain ecclesiastical discipline, especially as to the marriage laws. Ingiltrud, wife of Count Boso, had left her husband for a paramour; Nicholas commanded the bishops in the dominions of Charles the Bald to excommunicate her unless she returned to her husband. As she paid no attention to the summons to appear before the Synod of Milan in 860, she was put under the ban.
The pope was also involved in a desperate struggle with the bishops of Lotharingia over the inviolability of marriage. King Lothair II, not having any children by his lawful wife Teutberga, had abandoned her to marry his mistress Waldrada. At the Synod of Aachen on 28 April 862, the bishops of Lotharingia 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 Teutberga, who took refuge in the court of Lothair's uncle,Charles the Bald, and appealed to the Pope. Upon this the pope brought the matter before his own tribunal. The two archbishops, Günther of Cologne and Thietgaud of Trier, both relatives of Waldrada, had come to Rome as delegates, and 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 Engelberga arranged a reconciliation 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 his efforts to bring about a reconciliation between Lothair and his lawful wife.
Another matrimonial case in which Nicholas interposed was that of Judith of Flanders, daughter of Charles the Bald, who had married Baldwin I, 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.
In the East, Nicholas was seen as trying to extend his papal power beyond what was canonical authority by asserting a "rulership" over the Church instead of the position of "highest honor among equals" accorded to the pope of Rome by the East. He contended that the Patriarch of Constantinople Ignatius was deposed in 857 and Photius raised to the patriarchal see in violation of ecclesiastical law. In a letter of 8 May 862 addressed to the patriarchs of the East, Nicholas called upon them and all their bishops to refuse recognition to Photius, and at a Roman synod held in April 863, he excommunicated Photius.
By the will of the Byzantine Emperor Michael III, Photius was elected lawfully and canonically in 858 according to the Church of Constantinople. This was affirmed later in 879 in a council regarded as ecumenical by some in the Orthodox Church. Ignatius’ elevation to the Patriarchate was declared to be uncanonical and Photius was acclaimed as properly elected as the new Patriarch. The Eastern Church resented Nicholas' pressing of the doctrine of papal primacy. This led to conflict between Constantinople and Rome over doctrinal issues such as the addition of the Filioque clause to the Nicene-Constantinopolitan Creed and territorial claims due to the Church of Constantinople's seizure of territory from the Roman Patriarchate in southern Italy and Sicily. A synod at Constantinople in 867 excommunicated Nicholas and rejected his claims of primacy, his efforts to convert Bulgaria to the obedience of the Roman Church, and the addition of the Filioque clause in parts of the Latin Church.
For a variety of reasons, Prince Boris I of Bulgaria became interested in converting to Christianity and undertook to do that at the hands of western clergymen to be supplied by King Louis the German of East Francia in 863. Late in the same year, the Byzantine Empire invaded Bulgaria as it suffered famine and natural disasters. Boris was forced to sue for peace. Because the majority of his people were still opposed to Christianity, he was secretly baptized according to the Byzantine rite. The Byzantine Emperor who became his godfather conceded territory in Thrace to him.
Unhappy with Byzantine influence and desiring an autocephalous status which Photius was unwilling to grant, Boris sent an embassy to Nicholas with 106 questions on the teaching and discipline of the Church in August 866. Nicholas answered these inquiries in his "Responsa Nicolai ad consulta Bulgarorum" (Giovanni Domenico Mansi, "Coll. Conc.", XV, 401 sqq.) and sent missionaries under the papal legate bishop Formosus (later Pope Formosus). Also in 866, Nicholas sent a letter to the Bulgarians ordering the burning of any books captured from the Muslims because they were deemed harmful and blasphemous. When Pope Adrian II rejected Boris's request that either Formosus or Deacon Marinus (later Pope Marinus I) be made Archbishop of Bulgaria, Boris began to look again towards Constantinople. In 870 a council of Constantinople granted the Church of Bulgaria autocephalous status and Greek priests were sent as missionaries; they were soon replaced by Bulgarians.
Nicholas encouraged the missionary activity of the Church. He sanctioned the union of the Sees of Bremen and Hamburg, and confirmed to St. Anschar, Archbishop of Bremen, and his successors the office of papal legate to the Danes, Swedes, and Slavs. In many other ecclesiastical matters, he issued letters and decisions, and he took active measures against bishops who neglected their duties.
In Rome, Nicholas rebuilt and endowed several churches, and constantly sought to encourage religious life. He 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).
After his death he was regarded as a saint and was canonized by his successor, Pope Adrian II. His cult was re-affirmed in 1630 by Pope Urban VIII. His feast day is observed on 13 November.
A question that is important in judging the integrity of 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. He perhaps had a general knowledge of the false decretals, but did not base his view of the law upon them and owed his knowledge of them solely to documents that 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.].
Nicholas decreed that the figure of the rooster should be placed on every church and has served as a religious icon and reminder of Peter's denial of Christ since that time, with some churches still having the rooster on the steeple today.