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Peter of Sebaste (ca. 340 – 391) was a bishop, taking his usual name from the city of his bishopric, Sebaste in Lesser Armenia. He is also known as Peter of Sebasteia.
His parents were Basil and Emmelia of Caesarea-in-Cappadocia and he was brother to St. Macrina the Younger and the two Cappadocian doctors, St. Basil of Caesarea and St. Gregory of Nyssa. He was the youngest of a large family, and Macrina, his eldest sister, exercised a great influence over his religious training, acting as his instructress in the way of Christian perfection, and directing him toward the spiritual and ascetic life. Renouncing the study of the profane sciences, he devoted himself to meditation on Holy Writ and the cultivation of the religious life. Shortly after his brother's elevation to the episcopal See of Caesarea, Peter received from him priestly ordination, but subsequently, withdrawing from active affairs, resumed the life of a solitary ascetic. He assisted his sister toward the attainment of her life's object, and aided her and her mother in their monastic establishment after his father's death (Gregory of Nyssa, "Vita s. Macrinae"). Upon his declaration of chastity, Peter became involved in the gradual transformation of his mother's household from a community of virgins to a cenobitic community of both women and men. He headed the male monastery while Macrina was responsible for the female community.
About 380‑381 he was elevated to the See of Sebaste in Armenia and, without displaying any literary activity, took his stand beside his brothers Basil and Gregory in their fight against the Arian heresy (Theodoret, "H.E.", IV, xxvii). In his life and episcopal administration he displayed the same characteristics as Basil. Linked together in the closest manner with his brothers, he followed their writings with the greatest interest. At his advice Gregory of Nyssa wrote his great work, Against Eunomius, in defense of Basil's similarly named book answering the polemical work of Eunomius. It was also at his desire that Gregory wrote the Treatise on the Work of the Six Days, to defend Basil's similar treatise against false interpretations and to complete it. Another work of Gregory's, On the Endowment of Man, was also written at Peter's suggestion and sent to the latter with an appropriate preface as an Easter gift in 397. We have no detailed information concerning his activity as a bishop, except that he was present at the Ecumenical Council of Constantinople in 381. After his death in 391 he was venerated as a saint. His feast is kept by Roman Catholics on 8‑9 January.