Eustathius of Sebaste was a bishop during the mid fourth century controversies associated with Arianism. Originally a disciple of the heretic Arius, his position wavered through the various shades of Arianism: Arian, semi-Arian, anomoen, and even the Nicene faith. A monk, he advocated an exaggerated form of ascetic monasticism that he brought to Armenia.
Eustathius was born about the year 300 in Cappodoian Caesarea. He apparently was the son of Eulalius who was bishop of Sebaste in the metropolis of Armenia. About 320, he studied under Arius in Alexandria. From Alexandria he moved to Antioch where he was refused ordination by Eustathius of Antioch for his Arian leanings.  However, he was ordained c. 331 by Eulalius, but was quickly defrocked for refusing to adopt clerical dress. Eustathius returned to Caesarea where he was ordained again by the Orthodox Bishop Hermogenes after he firmly stated a belief in the Nicene faith.
After Bp. Hermogenes' death, Eustathius moved to Constantinople where he became a disciple of Eusebius. But, about 342, Eusebius deposed him again, this time apparently for some act of unfaithfulness to his duties. He again returned to Caesarea where by carefully concealing his Arian leanings, he commended himself to Bp. Dianius.
Apparently during the time before he became bishop of Sebaste, Eustathius engaged in theological discussions with Basil of Ancyra and the Anomoean Aetius, whom Basil regarded as in some sense a pupil of Eustathius. Apparently during this period, while a priest, he became a monk, a practice not common at the time, and introduced ascetic monasticism to Armenia. The practices in his cenobitic communities for both men and women forbade marriage for anyone, no communication with married priest, and taught that married people cannot be saved. The garments worn in the communities were of course materials with girdles and sandals of undressed hides. In 340, his monastic followers were condemned by a synod at Gangra for their exaggerated asceticism. 
About the year 357, Eustathius became bishop of Sebaste through the Arians. It was about this time that St Basil of Caesarea, who was himself entering upon a monastic life, became attracted by Eustathius' reputation as a leader of monasticism. While Eustathius was anything but orthodox, Basil continued a relationship with Eustathius until about 372.
Eustathius continued to waver among the varieties of Arianism, signing all manner of heretical and contradictory formulas of faith. In 385, he was deposed from the see of Sebaste by a synod at Melitene and succeeded by Meletius, although his semi-Arian followers remained loyal to him. His deposition seemed not to effect his standing as he was asked by the semi-Arian bishops to be at their synod at Ancyra to oppose the spread of Anomoean doctrines. At the Council of Seleucia on September 27, 359, Eustathius played a prominent place in the indecisive proceedings. He was among the group of bishops, who after the council, were sent to Constantinople to present their case before Emperor Constantius. Eustathius led their case against the Arians, presenting their formula of the the dissimilarity of the Father and Son, when the Arian delegation from the Council of Rimini, to the liking of Constantius, announced their position proscribing the Homoousion formula. With this turn Eustathius and his party were compelled to agree which ended the council of which St. Jerome wrote: "The whole world groaned and was astonished to find itself Arian." 
Constantius then, in January 360, called a council in Constantinople at which Acacius of Caesarea presided that deposed and banished Eustathius and a number of bishops including Cyril of Jerusalem, Basil of Ancyra, and Eleusius of Cyzicus. The council used Eustathius' previous deposition by Eulalius as sufficient cause and did not allow Eustathius to defend himself.
With the death of Constantius in 361, Eustathius and the other banished bishops were recalled by Emperor Julian. With his return, Eustathius immediately repudiated his signature on the creed of the Council of Rimini and the rejection of pure Arianism. As reported by Sozomen, Eustathius joined with others in a number of synods in which he condemned the supporters of Acacius, denounced the creed of the Rimini, and asserted that Homoiousion was the true formula over that of the Homoousion and the Anomoeon of Aetius and his followers. 
The accession of Valens as emperor in 364 change the landscape. Arianism again assumed ascendency. The party of semi-Arians, including Eustathius, now called Macedonians as they denied the divinity of the Holy Spirit, at a council chaired by Eleusius, repudiated the Acacian council of 360 at Constantinople and the creed of the Rimini council, renewed the semi-Arian creed of Antioch, and deposed Eudoxius and Acacius.  This raised the ire of Valens who required them to hold communion with Eudoxius, which they refused. Now under loss of their sees and banishment, the Macedonians, with Eustathius, journeyed to Rome to plead with the emperor Valentinian and Pope Liberius. Finding only a reluctant Liberius, they agreed to sign a written adherence to the Nicene creed and Homoousion.  However, no mention was made to the Macedonian heresy concerning the Holy Spirit that Eustathius subscribed to.
Returning to the East with letters of communion as proof of their orthodoxy, Eustathius and his fellow-delegates were received as the rightful bishops of their sees.  However, Valens, who was committed to Arianism, issued a edict expelling all bishops who had been restored by Julian. In reaction to save himself, Eustathius signed a formula formed around Homoiousian that also denied the divinity of the Holy Spirit.
Eustathius expressed great joy at the election of Basil of Caesarea to the episcopate in 370 and expressed an earnest desire to be of service to him. But, as Basil learned, these fellow-helpers of Eustathius turned out to be spies of Basil's actions and words that were all turned into malevolent senses when reported to Eustathius.  These Eustathius turned on his former associate, charging him with Apollinarian and other heretical views, and encouraged the clergy of Basil's diocese to form a rival communion.
Seeing an ascendency in Arianism, Eustathius, who had alienated many Arians, sought their recognition and continued to harass Basil until Basil's death in 379. Eustathius's death followed the next year, in 380, and was succeeded by Basil's brother Peter.
- ↑ Athan. Solit. p. 812
- ↑  The Council of Gangra
- ↑ Jerome, Dialogue Against the Luciferians, 19.
- ↑ H. E. v. 14
- ↑ Socr. H. E. iv. 2-4; Soz. H. E. vi. 7
- ↑ Socr. H. E. iv. 12; Soz. H. E. vi. 11
- ↑ Soz. l.c.; Basil. Ep. 244 , ¬ß 5
- ↑ Basil.ib. 223 , ¬ß 3