VIII.--St. Basil and Eustathius.
It was Basil's doom to suffer through his friendships. If the fault lay with himself in the case of Gregory, the same cannot be said of his rupture with Eustathius of Sebaste. If in this connexion fault can be laid to his charge at all, it was the fault of entering into intimacy with an unworthy man. In the earlier days of the retirement in Pontus the austerities of Eustathius outweighed in Basil's mind any suspicions of his unorthodoxy.  Basil delighted in his society, spent days and nights in sweet converse with him, and introduced him to his mother and the happy family circle at Annesi.  And no doubt under the ascendency of Basil, Eustathius, always ready to be all things to all men who might be for the time in power and authority, would appear as a very orthodox ascetic. Basil likens him to the Ethiopian of immutable blackness, and the leopard who cannot change his spots.  But in truth his skin at various periods shewed every shade which could serve his purpose, and his spots shifted and changed colour with every change in his surroundings.  He is the patristic Proteus. There must have been something singularly winning in his more than human attractiveness.  But he signed almost every creed that went about for signature in his lifetime.  He was consistent only in inconsistency. It was long ere Basil was driven to withdraw his confidence and regard, although his constancy to Eustathius raised in not a few, and notably in Theodotus of Nicopolis, the metropolitan of Armenia, doubts as to Basil's soundness in the faith. When Basil was in Armenia in 373, a creed was drawn up, in consultation with Theodotus, to be offered to Eustathius for signature. It consisted of the Nicene confession, with certain additions relating to the Macedonian controversy. 
 Ep. ccxiii. S: 3. He had been in early days a disciple of Arius at Alexandria.
 Id. S: 5.
 Ep. cxxx. S: 1.
 cf. Ep. ccxliv. S: 9. Fialon, Et. Hist. 128.
 Ep. ccxii. S: 2. cf. Newman, Hist. Sketches, iii. 20.
 Ep. ccxliv. S: 9.
 Epp. cxxi., ccxliv.
Reference address : https://www.elpenor.org/basil/life-works.asp?pg=28