Semi-Arianism: Almost Orthodox, Off by One Letter
The Son has a 'similar' substance to the Father — not identical, but close.
The Story
The most consequential theological debate in the history of Christianity came down to a single letter. The Greek letter iota — the smallest letter in the alphabet, barely a brushstroke — was the difference between orthodoxy and heresy, between the faith of the Nicene Creed and the position condemned by the councils.
Homoousios — “same substance.” This was the word the Council of Nicaea chose in 325 to describe the relationship between the Father and the Son. The Son is of the same substance as the Father. Same stuff. Same being. Fully God in exactly the way the Father is God.
Homoiousios — “similar substance.” This was the word favored by a large and sophisticated party of bishops who thought Nicaea had gone too far. The Son is like the Father. Similar in substance. Very close. Perhaps as close as a created being could possibly be to the uncreated God. But not identical. Not the same.
One iota. One letter. And the entire future of Christian theology hung on it.
The semi-Arian position emerged in the decades after Nicaea as a middle way in the Trinitarian controversies that consumed the fourth-century church. On one side stood the full Arians, followers of the Alexandrian priest Arius, who taught that the Son was a creature — the first and greatest of God’s creations, but a creation nonetheless, who “there was when he was not.” On the other side stood the Nicene party, led by Athanasius of Alexandria, who insisted that the Son was co-eternal and co-equal with the Father, of the same substance, God from God.
The semi-Arians tried to split the difference, and for several decades they very nearly won.
The leader of the semi-Arian party was Basil of Ancyra (modern Ankara, Turkey), a bishop and former physician who emerged as the chief theologian of the homoiousian position after the death of the more moderate Eusebius of Caesarea. Basil argued that calling the Son homoousios with the Father was dangerous because it could slide into Sabellianism — the heresy that Father, Son, and Spirit are merely different modes or masks of one divine person. If Father and Son are of identical substance, how do you distinguish them as persons? Better to say “similar substance,” which preserves both the Son’s divinity and the Father’s distinctness.
This was not a stupid argument. It was, in fact, a serious theological concern that the Nicene party would eventually need to address. The Cappadocian Fathers — Basil of Caesarea, Gregory of Nazianzus, and Gregory of Nyssa — would later develop the language of “one substance, three persons” (mia ousia, treis hypostaseis) precisely to resolve the tension that the semi-Arians had identified.
The political situation was chaotic. Emperor Constantius II (337-361) favored the semi-Arian position and used imperial power to promote it. Athanasius was exiled five times. The Council of Seleucia (359) and the Council of Ariminum (359) — twin councils convened by Constantius to settle the dispute — produced a vaguely worded compromise that the homoiousians could accept but the strict Nicenes could not. Jerome later wrote that at Ariminum, “the whole world groaned and was astonished to find itself Arian.”
Jerome was exaggerating, but not by much. Through the 350s and into the 360s, the semi-Arian position was arguably the dominant theology of the Eastern church. It had imperial support, sophisticated theologians, and the attractive virtue of appearing moderate. Nicaea looked like one extreme; full Arianism looked like the other; homoiousios looked like wisdom.
The semi-Arians were squeezed from both sides. The radical Arians — the Anomoeans, led by Aetius and Eunomius — pushed the Arian position to its logical extreme, arguing that the Son was unlike (anomoios) the Father in substance. This horrified the semi-Arians, who genuinely believed in the Son’s divinity (just not his identical divinity). Meanwhile, the Nicene party, strengthened by the Cappadocian Fathers, was developing a more nuanced version of homoousios that addressed the semi-Arian concerns about Sabellianism.
Athanasius himself recognized that many semi-Arians were closer to orthodoxy than they realized. In his De Synodis (359), he wrote that those who accepted homoiousios were “brothers who mean what we mean, and dispute only about the word.” He reached out to them, arguing that “similar in substance” inevitably implied “same in substance” — if two things are similar in their deepest nature, they must share that nature. The difference was linguistic, not theological.
The Council of Ancyra (358), led by Basil of Ancyra, formally articulated the semi-Arian position and anathematized both the Nicene homoousios and the radical Arian anomoios. But the council’s influence was short-lived. Emperor Constantius died in 361, and the brief reign of Julian the Apostate (who was pagan and did not care which Christians were right) paradoxically helped the Nicene cause by removing imperial pressure for compromise.
The decisive moment came at the First Council of Constantinople in 381, convened by Emperor Theodosius I, who was firmly Nicene. The council reaffirmed the Nicene Creed, expanded it into the form still recited in churches today (the Niceno-Constantinopolitan Creed), and definitively rejected both semi-Arianism and full Arianism. The homoiousios party was finished as a theological movement, though many of its adherents quietly accepted the Nicene formula once the Cappadocian Fathers had clarified that homoousios did not entail Sabellianism.
What the Council Actually Said
The original Nicene Creed (325) declared the Son to be:
“God from God, Light from Light, true God from true God, begotten not made, homoousios [of one substance] with the Father.”
The Third Council of Sirmium (357), sometimes called the “Blasphemy of Sirmium” by its opponents, went in the other direction and banned the use of both homoousios and homoiousios:
“Since some or many persons were disturbed by questions of substance, called in Greek ousia… there ought to be no mention at all of ousia since it is not contained in the divine Scriptures.”
The Council of Constantinople (381) reaffirmed and expanded the Nicene Creed, establishing the Son as:
“The only-begotten Son of God, begotten of the Father before all worlds; Light of Light, very God of very God; begotten, not made, being of one substance with the Father.”
Athanasius, reaching out to the semi-Arians, wrote:
“Those who accept the homoiousios are not far from accepting the homoousios. For to say ‘similar in substance’ is to say ‘the same in substance’… For silver is similar in substance to gold, but it is not the same. But the Son is not only similar to the Father but inseparable from the substance of the Father.”
Why You Might Accidentally Believe This
If you have ever said something like “Jesus is divine, but the Father is… more divine” — if you have an instinct that there is some kind of hierarchy within the Trinity, that the Father is the original and the Son is a (very exalted) derivative — you are in semi-Arian territory.
If the idea that Jesus is exactly the same substance as the Father strikes you as confusing or even polytheistic — if you find yourself thinking “but there’s only one God, and that’s the Father, and Jesus is… something else, something very high but not quite that” — you are closer to Basil of Ancyra than to the Nicene Creed.
Many Christians functionally operate with a semi-Arian Christology without knowing it. They pray to the Father as the “real” God and think of Jesus as God’s greatest representative, God’s perfect agent, God in human form but not God in the absolute sense. Surveys consistently suggest that a significant number of self-identified evangelicals hold views that are closer to semi-Arianism than to Nicaea.
The instinct behind semi-Arianism is deeply human: it preserves monotheism while honoring Christ. It avoids the paradox of saying that the carpenter from Nazareth is the same God who created the universe. It makes the Trinity less mind-bending by introducing gradation. The problem is that the councils concluded this gradation fatally undermines the gospel.
The Strongest Case For This View
The semi-Arian case was never trivial. It rested on genuine concerns.
First, the word homoousios is not found in Scripture. The semi-Arians were not wrong to point this out. The Bible never says the Son is “of the same substance” as the Father. The term was borrowed from Greek philosophy and applied to the biblical data, and its meaning was disputed even among those who used it. Nicaea imposed an extra-biblical word as a test of orthodoxy, and many bishops were uncomfortable with this.
Second, the subordination of the Son to the Father has genuine biblical support. Jesus says “the Father is greater than I” (John 14:28). He prays to the Father. He says “I do nothing on my own authority” (John 8:28). He is “sent” by the Father. Paul calls God “the head of Christ” (1 Corinthians 11:3). The semi-Arians argued that this language of derivation and subordination is best explained by a similar but not identical substance — the Son is from the Father, like the Father, but not simply the same as the Father.
Third, the concern about Sabellianism was legitimate. If Father and Son share identical substance, how are they distinct? The semi-Arians worried that homoousios collapsed the persons of the Trinity into a single undifferentiated being. This concern was serious enough that the Cappadocian Fathers had to develop new philosophical vocabulary — the distinction between ousia (substance) and hypostasis (person) — to address it.
Fourth, Basil of Ancyra and many semi-Arians were devout Christians who worshiped Christ and affirmed his divinity. They were not trying to diminish Christ; they were trying to describe his relationship to the Father with precision, and they thought homoiousios was more precise than homoousios.
The Strongest Case Against
The case against semi-Arianism was made most forcefully by Athanasius, and it cuts to the heart of Christian soteriology: if the Son is not fully God — not of the same substance as the Father — then he cannot save.
Athanasius’s argument is elegant and devastating. Only God can save humanity. If the Son is a creature, however exalted — if there is any gap between the Son’s substance and the Father’s — then the Son is on our side of the Creator-creature divide and cannot bridge it. Salvation requires that God himself enters into human nature, takes on human death, and defeats it from within. A being who is similar to God but not God cannot do this. “Similar substance” is not enough. Redemption requires identity of substance.
Gregory of Nazianzus pressed the point: “What has not been assumed has not been healed.” If the Son is not fully divine, his assumption of human nature does not fully heal that nature. Soteriology demands full divinity.
The linguistic argument also collapses under scrutiny. Athanasius pointed out that if two things are “similar in substance,” they must share that substance — otherwise, in what sense are they similar? The difference between homoousios and homoiousios is either meaningless (in which case, use the clearer term) or it introduces a real gap between Father and Son (which is theologically catastrophic). There is no stable middle ground.
The semi-Arian position also suffered from political instability. It required imperial support to maintain, and when that support evaporated after Constantius’s death, the theological inadequacy of the middle position became apparent. The semi-Arians could not hold the center because the center did not exist: either the Son is God or he is not. “Almost God” is not a coherent category.
Finally, the semi-Arians could not explain how a being of “similar substance” to God was worthy of the worship that Christians universally offered to Christ. If the Son is not of the same substance as the Father, then worshiping the Son is idolatry — worshiping a creature. The church worshiped Christ from the beginning; this worship is only justified if Christ is fully divine.
What the New Testament Actually Says
The New Testament presents a complex picture that both sides could plausibly claim.
The subordination texts are real. Jesus says “the Father is greater than I” (John 14:28). He is “sent” by the Father (John 3:17). He does not know the hour of his return (Mark 13:32). He prays to the Father and submits to the Father’s will (Luke 22:42). Paul calls God the Father “the God and Father of our Lord Jesus Christ” (Ephesians 1:3), implying a relationship of derivation.
But the equality texts are equally real. John 1:1 says the Word “was God” — not “was like God” or “was similar to God.” John 10:30: “I and the Father are one.” Colossians 2:9: “In him the whole fullness of deity dwells bodily.” Philippians 2:6 says Christ was “in the form of God” and did not regard “equality with God” as something to be grasped. Hebrews 1:3 calls the Son “the exact imprint of God’s nature” — the Greek word charakter suggests a precise representation, not an approximate one.
The worship of Christ in the New Testament is striking. Thomas calls the risen Jesus “my Lord and my God” (John 20:28). The book of Revelation gives Christ worship that, in the Old Testament, is reserved for God alone. Paul applies Old Testament texts about YHWH directly to Jesus (Philippians 2:10-11, echoing Isaiah 45:23).
The honest assessment: the New Testament affirms both the Son’s derivation from the Father and the Son’s full divinity. The tension between these affirmations is real, and it is the tension that the fourth-century councils were trying to resolve. The semi-Arians took the subordination texts seriously; the Nicene party took the equality texts seriously. The conciliar solution — homoousios combined with a real distinction of persons — was an attempt to hold both sets of texts together. Whether it succeeded depends on whether you find the Cappadocian distinction between ousia and hypostasis philosophically coherent or an elegant hand-wave.