Evagrius Ponticus of Pontus

[The heritics] ought to be confessing that the Father is God, that the Son is God, and that the Holy Spirit is God, just as is taught by the divine words and by those who have a better understanding of the Scriptures. To those who accuse us of a doctrine of three Gods, let it be stated that we confess one God, not in number but in nature. For all that is said to be one numerically is not to be one absolutely, nor is it simple in nature. It is universally confessed, however, that God is simple and not composite. [Apud Bas. Ep. 8,2]

In accord with the true teaching we do not speak of the Son as being either like or unlike the Father. Each of these terms is equally impossible; for likeness and unlikeness are predicated in respect to quality, and divinity is not restricted by quality. We, however, confess the identity of nature, accept the consubstantiality, and shun any compositness in the Father, God according to substance, who begot the Son, God according to substance. It is from the fact of their both being God according to substance that their consubstantiality is shown. [Apud Bas. Ep. 8,3]

You call the Holy Spirit a creature. And every creature is the servant of its creator. "All things," it is said, "serve You" (Ps.118[119]:91). But if He is a servant, His holiness is acquired; and anything which acquires its holiness is capable also of evil. It is because the Holy Spirit is holy in substance, however, that He is called "the font of sanctification" (See Rom. 1:4). The Holy Spirit, therefore, is not a creature. And if not a creature, He is consubstantial with God. [Apud Bas. Ep. 8,10]

"Letters" , in The Faith of the Early Fathers, Volume 2, pages 1-2.

Brief Biography: 345 - 399. Born at Ibora in the Pontus, was ordained a lector by St. Basil the Great and a deacon by St. Gregory of Nazianz.