Baring-Gould’s sources: Roman Martyrology and Greek Menæa. The life of S. Peter occurs in that of his sister, S. Macrina, written by his brother, S. Gregory of Nyssa. He is also spoken of by Socrates, Theodoret, and Philostorgius.
The family of which S. Peter was descended was very ancient and illustrious, as we are informed by S. Gregory Nazianzen. It has become famous for its saints, for three brothers were at the same time eminently holy bishops, S. Basil, S. Gregory of Nyssa, and S. Peter of Sebaste; and their elder sister, S. Macrina, was the spiritual mother of many saints. Their father and mother, S. Basil the elder, and S. Emilia, were banished for their faith in the reign of Galerius [Diocletian’s co-emperor in the East and driving force of the Great Persecution] Maximian [Diocletian’s co-emperor in the West, 286–305], and fled into the deserts of Pontus [a region on the Black Sea coast of Turkey]; they are commemorated in the Roman martyrology on May 30th. The grandmother of S. Peter was S. Macrina the elder, who had been instructed in the way of salvation by S. Gregory the Wonder-worker. S. Peter of Sebaste, was the youngest of ten children; he lost his father whilst still an infant, and was therefore brought up by his mother and sister. When the aged Emilia was dying, she drew her two children—the only two who were present—to her, and taking their hands, she looked up to heaven, and having prayed God to protect, govern, and sanctify her absent children, she said, "To Thee, O Lord, I dedicate the first-fruits; and the tenth of my womb. This, my first-born, Macrina, I give thee as my first-fruits; and this, my tenth child, Peter, I give thee as my tithe. They are thine by law, and thine they are by my free gift. Hallow, I pray thee, this my first-born daughter, and this my tenth child, and son." And thus blessing them, she expired, says S. Gregory Nyssen. S. Emilia had founded two monasteries, one for men, the other for women; the former she put under the direction of her son Basil, the latter under that of her daughter Macrina. Peter, whose thoughts where wholly bent on cultivating the seeds of piety sown in his heart, retired into the house governed by his brother, situated on the bank of the river Iris; and when S. Basil was obliged to quit that post in 362, he left the abbacy in the hands of S. Peter, who discharged this office for several years with great prudence and virtue. Soon after S. Basil was made Bishop of Cæsarea, in Cappadocia [a region of central Turkey], in 370, he promoted his brother Peter to the priesthood. His brother, S. Basil, died on Jan. 1st, a.d. 379, and Eustathius, Bishop of Sebaste, an Arian [followers of Arius, who taught that the Son is a created being, not fully God] and a furious persecutor of S. Basil, died soon after. S. Peter was consecrated in his room, in 380, to root out the Arian heresy in that diocese, where it had taken deep hold. In 381, he attended the general council held at Constantinople, and joined in the condemnation of the Macedonian heresy. His death happened in summer, about the year 387, and his brother, S. Gregory of Nyssa, mentions that his memory was honoured at Sebaste by an anniversary solemnity. "Peter," says Nicephorus (lib. ii. c. 44), "who sprang from the same parents as Basil, was not so well-read in profane literature as his brother, but he was not his inferior in the splendour of his virtue."
