Nicholas I Mystikos was Ecumenical Patriarch of Constantinople across two terms, from 901 to 907 and again from 912 until his death in 925. By tradition he was born around 852 to a prominent family of southern Italian (Roman) origin and was a relative and student of Patriarch Photios the Great. After Photios's fall from imperial favor in 886 Nicholas withdrew to monastic life, but Emperor Leo VI later recalled him to court and gave him the office of mystikos, a senior secretarial or judicial dignity from which his epithet derives. He was raised to the patriarchal throne, succeeding Anthony II, on 1 March 901.
His first patriarchate was dominated by the dispute known as the tetragamy, or fourth-marriage controversy. Emperor Leo VI, having lost three wives without a surviving male heir, wished to marry his mistress Zoe Karbonopsina to legitimize their son, the future Constantine VII. Eastern canon law strongly disapproved of a fourth marriage, and Nicholas refused to sanction it. He reluctantly permitted the baptism of the imperial child but, when the marriage was nonetheless celebrated by a cooperating priest, he barred the emperor from the church. His refusal to consult Pope Sergius III in the matter contributed to his deposition on 1 February 907, after which he was exiled to a monastery and replaced by Euthymius.
After Leo VI's death Nicholas was restored to the patriarchate in 912 under the emperor's brother and successor, Alexander, and he became the leading figure in the regency for the boy-emperor Constantine VII following Alexander's death in 913. As regent he conducted the empire's foreign policy at a perilous moment, negotiating with Simeon I of Bulgaria, but these concessions proved unpopular and by 914 Zoe Karbonopsina had displaced him from the regency. He later allied with Romanos I Lekapenos, was reconciled with the partisans of the rival patriarch Euthymius, and in a synod of 920 promulgated the Tomos of Union, which settled the tetragamy question by ordinarily limiting Christians to three marriages while making allowance for Leo VI's case for the good of the state.
Nicholas was an able administrator and a learned man whose extensive correspondence with foreign rulers and Byzantine notables survives, along with a homily occasioned by the Arab sack of Thessalonica in 904. He died in 925 and was buried at the Monastery of Galakrinon. The Orthodox Church venerates him as a confessor of canonical order against imperial pressure, commemorating him on May 16.
