“[We] must anathematize those who call the Holy Spirit a creature, those who think so, and those who do not confess that he is holy by nature, as the Father and Son are holy by nature, but who regard him as alien to the divine and blessed nature. A proof of orthodox doctrine is the refusal to separate him from the Father and Son (for we must be baptized as we have received the words, and we must believe as we are baptized, and we must give honour as we have believed, to the Father, Son and Holy Spirit), and to withdraw from the communion of those who call the Spirit a creature since they are clearly blasphemers. It is agreed (this comment is necessary because of the slanderers) that we do not say that the Holy Spirit is either unbegotten for we know one unbegotten and one source of what exists, the Father of our Lord Jesus Christ, or begotten, for we have been taught by the tradition of the faith that there is one Only-Begotten. But since we have been taught that the Spirit of truth proceeds from the Father we confess that he is from God without being created. (Basil, Letter 125.3).

In Basil’s thinking, since the Spirit is holy without qualification, he cannot be a creature and must be indivisibly one with the divine nature. The confession of this unity is both the criterion of orthodoxy and the basis upon which communion can be terminated with those who affirm that the Spirit is a creature. This pneumatological position thus defines the precise limits beyond which Basil was not prepared to venture, even for a friend such as Eustathius.

Another meeting was arranged for the autumn of 373, at which Eustathius was to sign this declaration in the presence of a number of Christian leaders. But on the way home from his meeting with Basil, Eustathius was convinced by some of his friends that Basil was theologically in error. For the next two years Eustathius crisscrossed what is now modern Turkey denouncing Basil, and claiming that the bishop of Caeasrea was a Modalist, one who believed that there were absolutely no distinctions between the persons of the Godhead.

Basil was so stunned by what had transpired that he kept his peace for close to two years. As he wrote later in 376, he was “astounded at so unexpected and sudden a change” in Eustathius that he able to respond. As he went on to say: “For my heart was crushed, my tongue was paralyzed, my hand benumbed, and I experienced the suffering of an ignoble soul…and I almost fell into misanthropy… [So] I was not silent through disdain…but through dismay and perplexity and the inability to say anything proportionate to my grief.” (Letter 244.4)

Finally, he simply felt that he had to speak. His words were those of the one most important books of the entire patristic period, On the Holy Spirit.

Quoting from Chapter XI, On the Holy Spirit - Basil speaks against those who deny the Holy Spirit.

"Who hath woe? Who bath sorrow?" For whom is distress and darkness? For whom eternal doom? Is it not for the trangressors? For them that deny the faith? And what is the proof of their denial? Is it not that they have set at naught their own confessions? And when and what did they confess? Belief in the Father and in the Son and in the Holy Ghost, when they renounced the devil and his angels, and uttered those saving words. What fit title then for them has been discovered, for the children of light to use? Are they not addressed as transgressors, as having violated the covenant of their salvation? What am I to call the denial of God? What the denial of Christ? What but transgressions? And to him who denies the Spirit, what title do you wish me to apply? Must it not be the same, inasmuch as he has broken his covenant with God? And when the confession of faith in Him secures the blessing of true religion. and its denial subjects men to the doom of godlessness, is it not a fearful thing for them to set the confession at naught, not through fear of fire, or sword, or cross, or scourge, or wheel, or rack, but merely led astray by the sophistry and seductions of the pneumatomachi? I testify to every man who is confessing Christ and denying God, that Christ will profit him nothing; to every man that calls upon God but rejects the Son, that his faith is vain; to every man that sets aside the Spirit, that his faith in the Father and the Son will be useless, for he cannot even hold it without the presence of the Spirit. For he who does not believe the Spirit does not believe in the Son, and he who has not believed in the Son does not believe in the Father. For none "can say that Jesus is the Lord but by the Holy Ghost," and "No man hath seen God at any time, but the only begotten God which is in the bosom of the Father, he hath declared him."

Such an one hath neither part nor lot in the true worship; for it is impossible to worship the Son, save by the Holy Ghost; impossible to call upon the Father, save by the Spirit of adoption.

Basil was clearly grieved having to write On the Holy Spirit. His desire to defend orthodoxy and bring the church back to a full understanding of the Holy Trinity trumped his desire for peace and communion. We can be thankful for Basil, this great church father who helped clearly define the Trinity.