The Council of Frankfurt Part 1: The Condemnation of Adoptionism

The Council of Frankfurt in 794 AD played a pivotal role in defending orthodox Christology against the growing influence of Adoptionism, a heresy that claimed Jesus, in his humanity, was "adopted" as the Son of God rather than being the eternal Son from the beginning. The council, convened by Charlemagne with the support of Pope Hadrian I, firmly rejected this teaching and reaffirmed the doctrine of Christ as one divine person with two natures—fully God and fully man from the moment of his incarnation.

The rejection of Adoptionism was not a political move but a theological necessity grounded in Scripture, the teachings of the early Church Fathers, medieval theologians, Protestant Reformers, and evangelical scholars throughout history. The Council of Frankfurt preserved the doctrine of Christ’s eternal sonship, ensuring that the Church remained faithful to the truth revealed in the Bible and the Nicene-Chalcedonian tradition.

The Rise of Adoptionism and the Need for the Council of Frankfurt

Adoptionism gained prominence in the late eighth century through the teachings of Elipandus, Archbishop of Toledo, and Felix, Bishop of Urgell. This movement arose in Muslim-controlled Spain, where Christians, living under Islamic rule, faced intense theological pressure to defend their belief in Christ’s divinity while emphasizing monotheism. In an attempt to reconcile Christ’s divine status with an extreme emphasis on his humanity, Elipandus and Felix argued that Jesus was not the Son of God by nature but was “adopted” as the Son at his baptism or resurrection.

Felix of Urgell developed a more systematic form of Adoptionism, stating that Christ was indeed the natural Son of God in his divine nature, but in his humanity, he was only an adopted son of God. This distinction was an attempt to preserve Christ’s divinity while also protecting his full humanity. However, it created an unbiblical and contradictory separation between Christ’s two natures, undermining the orthodox understanding of the hypostatic union, the doctrine that Christ is one person with two natures.

Elipandus of Toledo further promoted Adoptionism, emphasizing that Jesus as a man was exalted by God to divine sonship through grace. His teachings were influenced by earlier Nestorian ideas, which drew a sharp distinction between Christ’s divine and human natures to the point of almost dividing them into two separate persons. This effectively reduced Jesus’ divine nature to something conferred rather than inherent, contradicting the Nicene teaching that Christ is eternally begotten of the Father.

Recognizing the severe theological danger of this movement, Pope Hadrian I condemned Adoptionism and sought the support of Charlemagne, who in turn gathered over 300 bishops at the Council of Frankfurt. The council decisively condemned Adoptionism as a heresy, reaffirming that Christ was not an adopted Son but the eternal Son of God.

The Biblical Foundation for Christ’s Eternal Sonship

The most fundamental reason for rejecting Adoptionism lies in the clear testimony of Scripture, which consistently affirms that Jesus Christ is the Son of God not by adoption, but by nature and from eternity.

John’s Gospel states, "In the beginning was the Word, and the Word was with God, and the Word was God" (John 1:1). This passage establishes that Christ, as the Word (Logos), was not created or later adopted but has always existed as God. The doctrine of the Incarnation in John 1:14 states, "And the Word became flesh and dwelt among us." This does not suggest that Christ was at any point only human and later deified, but rather that the eternal Word took on human flesh while remaining fully divine.

Paul’s letter to the Philippians also affirms Christ’s pre-existence and divinity. "Though he was in the form of God, he did not count equality with God a thing to be grasped, but emptied himself, taking the form of a servant, being born in the likeness of men" (Philippians 2:6-7). The Greek phrase "being in the form of God" (ἐν μορφῇ Θεοῦ ὑπάρχων) indicates that Christ existed in divine nature before his incarnation. The passage does not support the idea that he later became divine through obedience or adoption but instead shows that his condescension was voluntary and rooted in his already existing divine status.

Likewise, the things Apollos writes in Hebrews affirm Christ’s eternal divine nature: "He is the radiance of the glory of God and the exact imprint of his nature" (Hebrews 1:3). If Christ were merely adopted as the Son, this passage would make no sense; rather, it confirms that Christ has always been fully God and fully the Son.

Jesus himself affirms his eternal sonship in John 17:5: "Father, glorify me in your own presence with the glory that I had with you before the world existed." This statement is irreconcilable with Adoptionism, as it proves that Christ’s sonship is not a later addition but an eternal reality.

Church Fathers’ Opposition to Adoptionism

The rejection of Adoptionism at Frankfurt was not an innovation but a reaffirmation of the Church’s continuous teaching on Christ’s divine sonship. The early Church Fathers consistently upheld the doctrine of Christ’s eternal sonship and refuted any suggestion that he became divine through adoption.

Irenaeus of Lyons, writing in the late second century, opposed any teaching that suggested Christ’s divinity was acquired rather than intrinsic. In Against Heresies, he declares, "The Son, eternally co-existing with the Father, from of old, did reveal the Father to the world" (Against Heresies, 4.6.7). This affirms that Christ’s sonship is eternal, directly contradicting the Adoptionist claim that Jesus became divine later.

Athanasius, the great defender of Nicene orthodoxy in the fourth century, also condemned any view that undermined Christ’s eternal divine sonship. In On the Incarnation, he writes, "The Word was not made man in order to become Son of God, but rather that he might make us sons of God" (On the Incarnation, 54). This distinction between Christ’s natural sonship and believers’ adoptive sonship contradicts Adoptionism.

Augustine of Hippo emphatically rejected any form of Adoptionism, stating, "Christ was not first a man and then became God, but he is always God, the eternal Son of the Father" (Sermon 186).

Protestant and Evangelical Affirmations of Christ’s Eternal Sonship

The Protestant Reformers and later evangelical theologians likewise upheld the doctrine of Christ’s eternal sonship and rejected any form of Adoptionism.

John Calvin, in Institutes of the Christian Religion, refuted Adoptionism: "Christ did not attain divine status by exaltation but was always the eternal Word" (Institutes, 2.14.1).

Martin Luther, in his Sermons on John, wrote, "Jesus is not made Son of God, but he is the Son from all eternity."

R. C. Sproul, in The Holiness of God, states, "The Son of God is eternally divine, not elevated to divinity through merit or adoption."

Modern evangelical theologians such as John MacArthur and D. A. Carson continue to affirm that Christ is the eternal Son of God and reject any teaching that implies he was at one time merely human before being exalted.

Conclusion

The Council of Frankfurt rightly condemned Adoptionism as a dangerous heresy that contradicted Scripture, Church tradition, and sound theology. Christ is not a man who became divine but the eternal Son of God, as affirmed by the entire history of Christian theology.

Athanasius summarized the significance of this doctrine when he wrote, "He became what we are so that we might become what he is—not by nature, but by grace." The Council of Frankfurt’s ruling was not just correct—it was essential for preserving the true gospel.