The Eunomian Heresy Explained: The Men Who Claimed to Know God’s Essence

Most heresies err by claiming too little for God. Eunomianism erred by claiming too much—about us. Its teachers held not merely that the Son was a creature, as Arius had taught, but that the divine essence itself was perfectly comprehensible to the human mind. Eunomius of Cyzicus, the movement’s great systematician, reportedly put it with breathtaking confidence: “God knows no more of his own substance than we do; nor is this more known to him, and less to us: but whatever we know about the Divine substance, that precisely is known to God; and on the other hand, whatever he knows, the same also you will find without any difference in us.”⁠¹

Against that astonishing claim the Cappadocian Fathers built the classic Christian theology of divine incomprehensibility. The fight with Eunomius forced the Church to say clearly that God can be truly known yet never fully comprehended—a principle that still anchors Catholic teaching on the mystery of God.

Where Eunomianism came from

Eunomianism was the second, more defiant wave of the Arian crisis. The Council of Nicaea had condemned Arius in 325 and made homoousios—“of one substance with the Father”—the touchstone of orthodoxy. For the next generation, most opponents of Nicaea worked by evasion, proposing softer formulas that avoided the word without confronting its substance. The Catholic Encyclopedia describes Eunomianism as “a phase of extreme Arianism prevalent amongst a section of Eastern churchmen from about 350 until 381,” one that “frankly returned to the fullest expression of the errors of Arius, and sought to defend it on the rationalizing basis of Aristotelean dialectics.”⁠²

Its founder was Aëtius, a self-made man—successively goldsmith, physician, and grammarian—who turned to theology under Arian influences at Antioch and Alexandria and was ordained deacon at Antioch in 350. Where the court bishops blurred distinctions, Aëtius sharpened them into syllogisms. His pupil and secretary Eunomius gave the system its lasting form. Installed as bishop of Cyzicus around 360 because, as the church historian Socrates records, “he was a person able by his eloquence to win over the minds of the multitude,” Eunomius so exasperated his flock with displays of dialectic that the people of Cyzicus drove him out of the city.⁠³

What Eunomius taught

The system rested on one definitional move. God’s essence, Eunomius held, is agennēsia—“unbegottenness” or “ingeneracy.” Not a property of God, not one attribute among many, but the very what-it-is of deity. From this single premise the rest followed with mechanical rigor.

If unbegottenness is God’s essence, then anything begotten is by definition not God. The Son, who is confessed by all Christians as “begotten,” therefore cannot share the Father’s essence. He is not merely subordinate; he is anomoios—unlike the Father in essence, a creature, the first and greatest of God’s works but a work nonetheless. Eunomianism thus out-Ariused Arius: where many fourth-century compromisers said the Son was “like” the Father (homoios) or even “like in essence” (homoiousios), the Anomoeans denied any likeness of essence at all.⁠⁴

The second consequence was stranger. If God’s essence just is a concept the mind can state—unbegottenness—then the mind that states it comprehends God. Divine mystery evaporates. Hence the boast preserved by Socrates: whatever God knows of his own substance, we know too, “without any difference.” Theology becomes a deductive science with no remainder.

The Cappadocian answer

The refutation of Eunomius occupied the greatest theological minds of the age. Basil of Caesarea wrote his Against Eunomius around 364, answering Eunomius’ Apology point by point; after Basil’s death, Eunomius answered back, and Gregory of Nyssa took up his brother’s defense in a massive Against Eunomius of his own.⁠⁵ Gregory of Nazianzus aimed the second of his Theological Orations squarely at Eunomian rationalism, arguing that even the angels do not comprehend the divine essence: to know that God is, is not to know what God is.⁠⁶

Three Cappadocian arguments proved decisive. First, “unbegotten” is a negative term—it tells us how God is not, not what God is; a privation cannot serve as a definition of essence. Second, human concepts of God arise by epinoia, reflective conception: they are true but partial, many windows on a reality none exhausts. Third, if “unbegotten” named the essence, then Father and Son could not even share a nature with each other in the way Scripture everywhere assumes when it calls the Son the radiance and exact imprint of the Father (Hebrews 1:3).

The dispute thus produced one of Christianity’s permanent intellectual achievements: a worked-out account of how language about God works. God is truly named from his operations and self-revelation, yet his essence remains beyond comprehension. The Catechism of the Catholic Church preserves exactly this balance: “Even when he reveals himself, God remains a mystery beyond words” (CCC 230).

Condemnation and afterlife

When the First Council of Constantinople met in 381, the Eunomians headed its list of condemned heresies. Canon 1 anathematized “the Eunomians or Anomoeans” before any other group—ahead of the Arians proper, the Pneumatomachians, and the Apollinarians.⁠⁷

A second canon tells us something the first does not. Canon 7, prescribing how converts from various heresies were to be received, ordered that Arians and Macedonians be admitted by anointing with chrism—but that Eunomians, “who are baptized with only one immersion,” be received by baptism, like pagans. Their baptism was not a defective version of the Church’s baptism; it was no baptism at all. Eunomius had revised the rite to match his theology, baptizing into the death of Christ with a single immersion rather than into the threefold Name. The Church’s sacramental verdict followed its trinitarian one: a sect that denied the consubstantial Trinity could not confer the Trinity’s baptism.⁠⁸

As a movement, Eunomianism did not long survive its founder’s generation. Theodosius’ legislation pressed it hard, its congregations splintered, and the Catholic Encyclopedia notes that as a sect it disappears from the record by the middle of the fifth century.⁠⁹ As a temptation, it has never disappeared: every rationalism that mistakes its concept of God for God himself is Eunomius redivivus.

Why it still matters

The Eunomian controversy settled, for Catholic theology, the relationship between reason and mystery. Against Eunomius the Church did not retreat into irrationalism—the Cappadocians argued harder and better than he did. What the Church rejected was the collapse of God into a definition. Reason ascends truly toward God; it never closes its hand around him.

That principle has liturgical teeth. The Church still baptizes by triple immersion or infusion into the threefold Name, still confesses the Son as “consubstantial with the Father” in the creed completed at Constantinople, and still teaches that the blessed in heaven see God face to face without ever comprehending him exhaustively. Each of those practices is, in part, a monument to the defeat of the man who said he knew God’s essence as well as God knows it.

Frequently Asked Questions

What is Eunomianism in simple terms?

Eunomianism was a radical form of Arianism, taught by Aëtius and Eunomius in the mid-fourth century, holding that God’s essence is “unbegottenness,” that the Son—being begotten—is therefore a creature utterly unlike the Father in essence, and that the divine essence is fully comprehensible to human reason. It was condemned by name at the First Council of Constantinople in 381.

How did Eunomianism differ from ordinary Arianism?

Arius taught that the Son was a creature, but most of his fourth-century heirs softened this, calling the Son “like” the Father. The Eunomians (also called Anomoeans, from anomoios, “unlike”) rejected all compromise: the Son is unlike the Father in essence. They also added a claim Arius never made—that God’s essence is perfectly knowable.

Who refuted Eunomius?

Basil of Caesarea (Against Eunomius, c. 364), his brother Gregory of Nyssa (a much longer Against Eunomius, written after Basil’s death), and Gregory of Nazianzus (the Theological Orations, especially the second). Their replies built the classic Christian doctrine of divine incomprehensibility.

Why did the Church rebaptize former Eunomians?

Because Eunomius altered baptism itself, using a single immersion rather than baptism into the threefold Name of Father, Son, and Holy Spirit. Canon 7 of Constantinople therefore treated Eunomian baptism as invalid and required converts to be baptized, while converts from most other heresies were received by anointing.

Is the claim that God can be fully understood still considered heretical?

The Catholic Church teaches that God can be known truly—even by natural reason—but never comprehended exhaustively: “Even when he reveals himself, God remains a mystery beyond words” (CCC 230). The Eunomian claim to comprehend the divine essence remains incompatible with Catholic teaching on the mystery of God.

---

Footnotes

1. 1. Socrates Scholasticus, Ecclesiastical History IV.7, trans. NPNF Second Series, Vol. 2, newadvent.org. Socrates introduces the quotation as what “Eunomius himself has the hardihood to utter in his sophistical discourses concerning the Deity.” The report is hostile but consistent with the epistemological optimism of Eunomius’ surviving Apology.

2. 2. Catholic Encyclopedia, “Eunomianism,” newadvent.org.

3. 3. Socrates, Hist. eccl. IV.7, newadvent.org; on Aëtius’ career (goldsmith, physician, grammarian, deacon at Antioch in 350), see the Catholic Encyclopedia, “Eunomianism.”

4. 4. On the doctrine of agennêsia as the divine essence and the anomoios conclusion, see R. P. C. Hanson, The Search for the Christian Doctrine of God: The Arian Controversy 318–381 (Edinburgh: T&T Clark, 1988), on the Neo-Arians, and J. N. D. Kelly, Early Christian Doctrines, 5th ed. (London: A&C Black, 1977), on the later Arianism.

5. 5. Gregory of Nyssa, Against Eunomius, trans. NPNF Second Series, Vol. 5, newadvent.org; Basil of Caesarea, Against Eunomius, trans. Mark DelCogliano and Andrew Radde-Gallwitz, Fathers of the Church 122 (Washington, DC: Catholic University of America Press, 2011).

6. 6. Gregory of Nazianzus, Oration 28 (Second Theological Oration), NPNF Second Series, Vol. 7, newadvent.org.

7. 7. Canon 1 of Constantinople (381), in Henry R. Percival, trans., The Seven Ecumenical Councils, NPNF Second Series, Vol. 14, and Norman P. Tanner, ed., Decrees of the Ecumenical Councils, vol. 1 (1990); the conciliar texts are online at Papal Encyclicals Online.

8. 8. Canon 7 of Constantinople (in the Greek numbering; generally regarded as a later addition to the council’s corpus, and presented as part of the fifth canon in New Advent’s arrangement), text in Percival, NPNF II.14, and at New Advent: Eunomians, “who are baptized with only one immersion,” are received “as heathen.” Theodoret (Haer. fab. IV.3) attributes the altered baptismal practice to Eunomius himself; Sozomen (Hist. eccl. VI.26) credits the “death of Christ” formula to his followers.

9. 9. Catholic Encyclopedia, “Eunomianism”: “as a sect it is not heard of after the middle of the fifth century.”