Eunomius, whom Ullmann calls one of the most interesting heretics of the Fourth Century, was by birth a Cappadocian, and slightly older than Gregory. As a young man he was a pupil and amanuensis of Aetius, by whom the Arian heresy was developed to its extreme results. The disciple never shrank from drawing the furthest logical conclusions from his master's premises, or from stating them with a frankness, which to those who regarded the premises themselves from which he reasoned as horrible blasphemies, seemed nothing less than diabolical in its impiety. So precisely did he complete and formulate his teacher's heretical tenets, that the Anomoean Arians were ever afterwards called Eunomians, rather than Aetians. They asserted the absolute Unlikeness of the Being of the Father and of the Son. Starting with the conception of God as Absolute Being, of Whom no Generation can be predicated, Unbegotten and incapable of Begetting, they went on to say that an Eternal Generation is inconceivable, and that the Generation of the Son of God must have had a beginning. Of course, therefore, the Arian conclusion followed, namely, that there was a time when the Son did not exist (en pote hote ouk en), and His Essence is altogether unlike that of the Unbegotten Father. Equality of essence and Similarity of essence, are alike untenable, from the mere fact that the one Essence is Unbegotten, and the other is Begotten. The Son, they said, is the First Creation of the Divine Energy, and is the Instrument by whom God created the world, and in this sense, as the Organ of creative power, may be said to be the Express Image and Likeness of the Energy of the Father. 
As they viewed the Holy Ghost as sharing the Divine Nature in an even remoter degree, as being only the noblest production of the Only-begotten Son, Eunomius was the first person heretically to discontinue the practice of threefold immersion in Holy Baptism. He also corrupted the Form of that Sacrament, by setting aside the use of the Name of the Father, Son, and Holy Ghost, and baptizing people "in the name of the Creator, and into the death of Christ." Therefore the Council of Constantinople ordered that converts from Eunomianism should be baptized, although those from other forms of Arianism were admitted into the Catholic Church by simple imposition of hands. Through the influence of the followers of Aetius, Eunomius became, in 360, Bishop of Cyzicus in Mysia, but he does not appear to have occupied the See very long. At any rate when Gregory came, in 379, to Constantinople, he was living in retirement near Chalcedon. All parties concur in representing him as a consummate Dialectician, but the Orthodox declared that he had turned Theology into a mere Technology. Readiness of Dialectic was the great characteristic of his Sect, and it was they who introduced into the Capital that bad spirit of theological disputatiousness which Gregory deplores in the first of these famous Orations. He also differed entirely from Gregory, not merely in the conclusions at which he arrived, but in the method by which he reached them; following the system of Aristotle, rather than of Plato, and using an exclusively intellectual method, while Gregory treated Religion as belonging to the entire man. The point at issue between them, besides this of the Interior relations of the Three Blessed Persons within the Godhead, was mainly the question as to the complete comprehensibility of the Divine Nature, which the Eunomians maintained, and Gregory denied. The latter argued that, while we have a sure conviction that God is, we have not a full understanding of What He is. He would not, however, exclude us from all knowledge of God's Nature, only he limits our capacity to so much as God has been pleased to reveal to us of Himself. "In my opinion," he says (Or. xxiv. 4), "it is impossible to express God, and yet more impossible to conceive Him—seeing that the thick covering of the flesh is an obstacle to the understanding of the truth." Similarly in the Fourth of these Orations (Or. xxx. 17) he says, "The Deity cannot be expressed in words. And this is proved to us, not only by arguments, but by the wisest and most ancient of the Hebrews, so far as they have given us reason for conjecture. For they appropriated certain characters to the honour of the Deity, and would not even allow the name of anything inferior to God to be written with the same letters as that of God, because to their mind it was improper that the Deity should even to that extent admit any of His creatures to a share with Himself. How then could they have admitted that the indivisible and separate Nature can be explained by divisible words?"
 Two terms borrowed from Holy Scripture (Heb. i. 3). But observe, borrowed with a difference—not "the Image of His Substance," which they would not admit, but of His "Energy," which is a very different conception.
Reference address : https://www.elpenor.org/gregory-nazianzen/introduction-theological-orations.asp?pg=2