NICHOLAS I., sometimes called The Great, and certainly the most commanding figure in the series of popes between Greg ory I. and Gregory VII., succeeded Benedict III. in April 858. According to the annalist Prudentius of Troyes, "he owed his election less to the choice of the clergy than to the presence and favour of the emperor Louis II. and his nobles"—who can hardly have foreseen with what ability and persistency the rights of the Holy See as supreme arbiter of Christendom were to be asserted even against themselves by the man of their choice. Of the previous history of Nicholas nothing is recorded. His pontificate of nine years and a half was marked by at least three memorable contests which have left their mark in history. The first was that in which he supported the claims of the unjustly degraded patriarch of Constantinople, Ignatius (q.v.) ; but two of its inci dents, the excommunication of Photius (q.v.), the rival of Igna tius, by the pope in 863, and the counter-deposition of Nicholas by Photius in 867, were steps of serious moment towards the permanent separation between the Eastern and the Western Church.
The second great struggle was that with Lothair (q.v.) the king of Lorraine (second son of the emperor Lothair I., and brother of the emperor Louis II.), about the divorce of his wife Theut berga or Thietberga. The pope not only quashed the whole pro ceedings against Theutberga, but excommunicated and deposed bishops Gunther and Thietgaud, who had been audacious enough to bring to Rome in person the "libellus" of the synod which had given judgment. The archbishops appealed to Louis II., then at Benevento, to obtain the withdrawal of their sentence by force; but, although he actually occupied the Leonine city (864), he was unsuccessful in obtaining any concession, and had to withdraw to Ravenna.
The third great ecclesiasical cause which marks this pontificate was that in which the right of bishops to appeal to Rome against their metropolitans was maintained in the case of Rothad of Soissons, deposed by Hincmar of Reims. In the course of the controversy with the great and powerful Neustrian archbishop papal recognition was first given (in 865) to the False Decretals, which had probably been brought by Rothad to Rome in the preceding year. (See DECRETALS.) Nicholas was the pope to whom Boris, the newly converted king of Bulgaria, addressed himself for practical advice in some of the difficult moral and social prob lems arising out of the transition from heathenism to Christianity. The pope's letter in reply to the hundred and six questions and petitions of the barbarian king is perhaps the most interesting literary relic of Nicholas I. now extant. He died on Nov. 13, 867, and was succeeded by Adrian II.
The epistolae of Nicholas I. are printed in Migne, Patrologia Lat. vol. 119, p. 769 seq. See F. Gregorovius, Rome in the Middle Ages, vol. iii. (Eng. trans., London, 1900-1902) ; H. Lammer, Nikolaus I. und die byzantinische Staatskirche seiner Zeit (Berlin, 1857) ; J. Roy, Saint-Nicolas I. (Paris, 1900) ; J. Richterich, Papst Nikolaus I. (Bern, 1903) ; A. Greinacher, Die Anschauungen des Papstes Nikolaus I. fiber das Verhilltnis von Staat und Kirche (1909) .