Eustathius, of Sebasteia

Eustathius of Sebasteia (c. 300 - c. 377) was bishop of Sebasteia in Armenia Minor and the most influential pioneer of organized asceticism in fourth-century Asia Minor. A charismatic but theologically restless figure, he championed a rigorous monastic discipline that profoundly shaped the young Basil of Caesarea, who long revered him as a spiritual mentor before their bitter and public rupture over the doctrine of the Holy Spirit. Often counted among the leaders of the Homoiousian ("semi-Arian") party, Eustathius shifted allegiances across the doctrinal controversies of his age, and his austere movement provoked the disciplinary canons of the Synod of Gangra, which condemned ascetic excesses associated with his followers. He survives chiefly through the letters and treatises of Basil, who both credits him as the formative influence on Cappadocian monasticism and denounces him as a doctrinal turncoat.