By the middle of the fifth century, a new wave of Christological debates became linked with the names of Eutyches, an archimandrite from Constantinople, and his supporter Dioscorus, St. Cyril’s successor to the episcopal throne of Alexandria. Eutyches spoke in terms of the complete ‘merging’ of the divinity with the humanity into a ‘single incarnate nature of God the Word’. ‘I confess that our Lord consisted of two natures before the union, but after the union I confess one nature’, said Eutyches.

The Fourth Ecumenical Council, convoked in 451 at Chalcedon, condemned Eutychian Monophysitism and proclaimed the dogma of ‘a single hypostasis of God the Word in two natures, divine and human’. According to the Council’s teaching, each nature of Christ preserves the fullness of its properties, yet Christ is not divided into two persons; He remains the single hypostasis of God the Word. This belief was expressed in the Council’s dogmatic definition: ‘...We confess one and the same Son, our Lord Jesus Christ, perfect in Divinity, perfect in humanity, truly God, truly human being, with a rational soul and body, consubstantial with the Father in His Divinity and consubstantial with us in His humanity.., one and the same Christ, the Son, the Only-begotten Lord, discerned in two natures without confusion, without change, without division, without separation’.

These clearly-defined formulas demonstrate the refinement and insight that theological thought had reached in the Christian Church by the fifth century. At the same time they show the caution with which the church Fathers used different terms and formulas in trying to ‘express the inexpressible’. Four terms were used to convey the union of the two natures (‘without confusion, without change, without division, without separation’), and each was strictly apophatic. The union of the divine and human natures in Christ is a mystery which transcends the intellect and no term is capable of describing it. What is spoken of with precision is how the natures are not united: this is to avoid heresies which could confuse, change and divide the natures. However, how the natures are united, remains concealed from human intellect.