Christology is a branch of theology that concerns Jesus Christ. The Church's official interpretation of divine revelation, and Christological doctrine in particular, had to be formally developed as a response to various erroneous teachings that arose from the earliest days of Christianity.
Arius, a presbyter in Alexandria, began teaching in 313 that the Son possessed neither the eternity nor the true divinity of God the Father. Reconstructing Arius's exact views is hard because little of his own work survives. The creed of Arian bishop Ulfilas, quoted in a letter by Auxentius of Milan, distinguishes God the Father, who is the only true God from Son of God, who is Lord/Master, and the Holy Spirit, the illuminating and sanctifying power.
Arius's opponents objected that, if the Son is not equal to God the Father, believers have no chance to reach a true union with God—salvation and supreme bliss—the ultimate goal of a Christian life: "...we all...are being changed into his likeness from one degree of glory to another" (2 Corinthians 3:18).
Many early Christians likely did not understand all the philosophical subtleties of Christological debates, but they were clearly aware of this goal. The Greek word epoptea—reception of bliss resulting from divine communion—even in pre-Christian times signified the highest stage in the spiritual life (a lower stage was called catharsis—a word which is often used today in other contexts).
On the other hand, the complexity of our world, its very existence, can be viewed, intuitively, as a reflection of the complexity of thought of its Creator, and as a reason for a certain complexity of our theological doctrines.
To account for Scripture’s presentation of the one God who is triune, Christian theology affirmed that there are three distinct divine persons who fully share the one, undivided divine nature. Nature referred to what an object is. A divine nature is what God is in his one, undivided essence, which we describe in terms of God’s attributes. A human nature is what constitutes humanity.
A person (Greek: hypostasis, Latin: persona) is an acting subject, nature is not. Yet, what is true of a person's nature is true of the person itself (known as "communication of attributes").
The Church dealt with Arianism in 325 at the Council of Nicaea, called by the Emperor Constantine. It resulted in the first uniform Christian doctrine, called the Nicene Creed. The Council decided against Arian overwhelmingly (of the estimated 250–318 bishops present, all but two agreed to sign the creed). The Council declared that Jesus is "begotten not made," "light from light," "true God from true God," and "of one being (or essence) with the Father."
Constantine issued an edict stating: "if any writing composed by Arius should be found, it should be handed over to the flames, so that not only will the wickedness of his teaching be obliterated, but nothing will be left even to remind anyone of him."
A different obstacle to attaining Christians' union with God would arise if there were separation between divinity and humanity in Christ, either because of an absence of union between His divine and human natures, or because He acted as two separate—divine and human—persons.
In the 5th century, a dispute arose between Nestorius, Archbishop of Constantinople and St. Cyril, Patriarch of Alexandria, in which Nestorius claimed that the term Theotokos ("Mother of God") could not be used to describe the Virgin Mary. He argued that God could not be born because the divine nature is unoriginate. That implied that Jesus was born in union with, but separate from and not strictly identifiable with, the Logos ("Word"), the second person of the Trinity.
Nestorius's opponents charged him with detaching Christ's divinity and humanity into two persons existing in one body (operating independently, at different times), thereby denying the reality of the Incarnation.
Nestorius' doctrine had brought him into conflict with other church leaders, most notably Cyril of Alexandria. Nestorius himself had requested the Emperor Theodosius II to convene a council of Christian bishops, hoping that it would prove his orthodoxy; the Council of Ephesus in fact condemned his teachings as heresy.
The Council decreed that Jesus was one person (hypostasis), possessing both a human and divine nature. The Virgin Mary was to be called Theotokos. The Council also confirmed the original Nicene Creed.
The Emperor Theodosius II had always championed Nestorius, but had been somewhat shaken by the reports of the Council. Communication with Constantinople was impeded by the friends of Nestorius there and at Ephesus. A letter was taken to Constantinople at last in a hollow cane, by a messenger disguised as a beggar. The envoys, whom the Council was eventually allowed to send to the Court, persuaded the emperor to accept its decisions. Anticipating his fate, Nestorius requested permission to retire to his former monastery. Cyril returned to Alexandria amid much joy.
About two years after Cyril of Alexandria's death in 444, an aged monk from Constantinople named Eutyches began teaching a subtle variation on the traditional Christology in an attempt to stop what he saw as a new outbreak of Nestorianism. The energy and imprudence with which Eutyches asserted his opinions led to him being misunderstood. Many believed that Eutyches was advocating a sort of reversal of Arianism, denying that Jesus was fully human.
At the Council of Chalcedon in 451, gathered church leaders affirmed the two natures of Christ, one human and one divine, each distinct and complete, united with neither confusion nor division. The famous Confession of Chalcedon states (note that Godhead is derived from Middle English godhede, "godhood", and unrelated to the modern word "head"):
We, then, following the holy Fathers, all with one consent, teach people to confess one and the same Son, our Lord Jesus Christ, the same perfect in Godhead and also perfect in manhood; truly God and truly man, of a rational soul and body; co-essential with the Father according to the Godhead, and co-essential with us according to the Manhood; in all things like unto us, without sin; begotten before all ages of the Father according to the Godhead, and in these latter days, for us and for our salvation, born of the Virgin Mary, the Mother of God, according to the Manhood; one and the same Christ, Son, Lord, only begotten, to be acknowledged in two natures, inconfusedly, unchangeably, indivisibly, inseparably; the distinction of natures being by no means taken away by the union, but rather the property of each nature being preserved, and concurring in one Person and one Hypostasis, not parted or divided into two persons, but one and the same Son, and only begotten God, the Word, the Lord Jesus Christ; as the prophets from the beginning [have declared] concerning Him, and the Lord Jesus Christ Himself has taught us, and the Creed of the holy Fathers has handed down to us.
All major branches of modern Christianity accept this Chalcedonian definition.