These were the years between the First Ecumenical Council (Nicea, 325) and the Second (Constantinople, 381), years in which it was uncertain whether the Church would stand by the declaration made at Nicea that the Logos (the "Word" -- see John 1:1) was fully God, equally with the Father, or seek a more flexible formula in the hope of reconciliation with the Arians, who declared themselves unalterably opposed to the Nicene wording. Basil had been ordained priest in 362 in order to assist the new Bishop of Caesarea, whom he succeeded in 370. (Since Caesarea was the capital, or metropolis, of the province of Cappadocia, its bishop was automatically the metropolitan of Cappadocia, which included about fifty dioceses (bishoprics). A metropolitan was roughly what we would call an archbishop, although in ancient terminology an "archbishop" was one step above a metropolitan.) By that time, an Arian emperor, Valens, was ruling. Basil made it his policy to try to unite the so-called semi-Arians with the Nicene party against the outright Arians, making use of the formula "three persons (hypostases) in one substance (ousia)," thus explicitly acknowledging a distinction between the Father and the Son (a distinction that the Nicene party had been accused of blurring), and at the same time insisting on their essential unity.
When the emperor Valens passed through Caesarea in 371, he demanded the theological submission of Basil, who flatly refused. The imperial prefect expressed astonishment at Basil's defiance, to which Basil replied, "Perhaps you have never met a real bishop before." Valens retaliated by dividing the province of Cappadocia into two provinces, with the result that the Arian Bishop of Tyana became metropolitan of the new province of Western Cappadocia. Basil responded by going political. He ramrodded his brother Gregory of Nyssa (9 May) and his friend Gregory of Nazianzus into bishoprics that they did not want, and for which they were totally unsuited, so that he would have the votes of those bishoprics when he needed them. (Neither Gregory ever quite forgave him for this.) His interests were not exclusively theological: he denounced and excommunicated those who owned houses of prostitution, he worked to secure justice for the poor against those who oppressed them, and he severely disciplined clergy who used their office to accumulate money or to live too well at the expense of the faithful.

His most famous writings include the Hexaemeron ("The Six Days"), a series of nine sermons on the days of creation, in which he speaks of the beauties of the created world as revelations of the splendor of God. His Against Eunomius defends the deity of Christ against an Arian writer, and his On the Holy Spirit speaks of the deity of the Third Person of the Trinity, and the rightness of worshipping Him together with the Father and the Son. In his Address To Young Men (originally written for his nephews), he urges Christians to make themselves acquainted with pagan philosophy and literature, arguing that this will often lead to a deeper understanding of Christian truth. His personality comes through most clearly in his letters, of which more than three hundred have been preserved. Some deal with points of theology or ethics, some with canon law, and many simply with everyday affairs. Ten times a year the Eastern churches use the Liturgy of St Basil rather than the more usual Liturgy of St John Chrysostom. It differs chiefly in having a more elaborate Anaphora (the prayer of consecration offered over the bread and wine), expressing some of his characteristic turns of thought, probably dating back to his time and used by him, and possibly composed by him personally.

Basil died in 379, shortly after the death in battle of the Arian Valens removed the chief threat to the Nicene faith to which Basil had devoted his life. He was mourned by the entire city, and the weeping crowds at his funeral included Christians, Jews, and pagans. He is counted (with the two Gregories) as one of the three Cappadocian Fathers, and (with Gregory of Nazianzus and John Chrysostom) as one of the Three Holy Hierarchs. In the West, he is reckoned (with Gregory of Nazianzus, John Chrysostom, and Athanasius) as one of the Four Greek (Eastern) Doctors of the Undivided Church. (The Four Latin (Western) Doctors are Ambrose, Jerome, Augustine, and Gregory the Great.)