"And the Word became flesh and dwelt among us, (and we beheld His glory, as of the only begotten of the Father), full of grace and truth." —St. John 1:14
Christology, in the narrow sense of the word, attempts to define the relation of the human to the divine in the person of Christ. Nicea promulgated that the Word who "became flesh" was of the same divine nature as God the Father (cf. John 10:30), and the universal belief of early Christendom was that Jesus was both divine and human. New Testament writers attributed to him a two-fold order of being, "according to the flesh" i.e. as man, and "according to the Spirit," i.e. as God.
The starting point is that Christ as a person is indivisibly one, simultaneously fully God and fully man. However, in the first three centuries of Christianity, important currents of thought flowed outside and around this starting point. Beliefs about the nature(s) of Christ range the gamut from Jesus is just a man (Ebionism) to Jesus is just a spirit (Docetism).
Ebionism arose during the second century. It attempted to solve the Christological problem by denying the divinity of Christ altogether: no divinity, no problem. The Ebionites were an offshoot of the Judaic branch of Christianity that, were it not for St. Paul's forceful defense of the equality of both the Jew and the Greek before the New Covenant (Galatians 3:28), might have saddled the Church with observance of the Jewish law. The Ebionites also rejected the Virgin birth, regarding Jesus as a man born from Joseph and Mary.
On the other side of the Christological coin there arose Docetism, which denied Jesus' humanity. Docetism held that Christ only "seemed" to be a man, and that He only "seemed" to suffer. These ideas are rooted in notions of divine impassibility: If Jesus is God, and God cannot suffer, can Jesus suffer? Since the answer to this question is assumed to be no, then Jesus could not have suffered as a man either. Justin Martyr felt the need to address this when he remarked, "there are some who declare that Jesus Christ did not come in flesh but only in spirit, [exhibiting] an appearance of flesh."
St. John states that the Word, the second person of the Godhead, became flesh and dwelt among us. He took flesh from the Virgin Mary and became at once everything man is except without sin (Hebrews 4:15). But at the same time, the "Word made flesh" had to be a real man: to assert that His experiences and sufferings as a man were only illusory was to deny His salvific work as the "Son of Man:" Christ could not have saved us were He not "in all points tempted like as we are." So orthodox apologists denied both Docetism and Ebionism. Christ is fully man and fully God.
Gnosticism was an early heresy which claimed to hold Christian doctrine in secret, contradicting Christ's response to the Sandrehin when asked about His doctrine:
"I spoke openly to the world; I ever taught in the synagogue, and in the temple, whither the Jews always resort; and in secret have I said nothing. Why askest thou me? Ask them which heard me, what I have said unto them: behold, they know what I said." (John 18:20-21).Gnostics argued that those who were uninitiated to their "secrets" could not be saved. Their notion of Christology was bizarre, even going so far as to assert that Jesus' body was made out of a "psychic substance." This runs afoul of 1 John 1:1, where St. John states that "our hands have handled" the Word of Life. It's hard to shake hands or give a pat on the back to a psychic substance. There were many other wild speculations in Gnostic circles, but I shall not list them here.
Even though Gnosticism more or less existed on the fringe, it came close to shipwrecking the central Tradition, that is, that Christ was fully man and fully God. That it did not was in large part due to the rule of faith that decreed that Jesus Christ had really become man, concisely expressed in St. John's first epistle:
"Hereby know ye the Spirit of God: Every spirit that confesseth that Jesus Christ is come in the flesh is of God: And every spirit that confesseth not that Jesus Christ is come in the flesh is not of God: and this is that spirit of antichrist, whereof ye have heard that it should come; and even now already is it in the world." - 1 John 4:2-3But the question still remained: given Christ is fully man and fully God, how was this union of the human and divine natures actually accomplished? The Christological theory which commanded the most support in the second century was called "Spirit Christology." By this is meant that in the historical Jesus Christ, the pre-existent Son of God, the second Person of the Trinity Who is divine Spirit (John 4:24), united Himself with human nature. This could take several forms, among them either that the pre-existent Spirit indwelt the man Jesus, or that the Spirit actually became man. Ignatius' Christology seems to conform to this second type. Jesus was fully and characteristically human, but was also born of the seed of David. That is, He was the of the seed of David according to the flesh, and the Son of God according to God's will and power. Jesus was the product of the perfect union of divine spirit with human nature in the womb of the Blessed Theotokos (i.e. Virgin Mary).
The West was quicker to formulate a mature Christology than the East. In part this was due to its possessing theologians of the caliber of Hippolytus and Tertullian The pattern the latter shaped was to prove of lasting significance, but let us begin with the former. Like his teacher Irenaeus, Hippolytus employed the Johannine model to explain the Incarnation: the Word was made flesh. Some of his writings seem to imply that the Logos simply assumed human flesh as an outward garment, for example when he compares Christ's humanity to a bridegroom's robe. But the true meaning comes out when he says that the "Logos became flesh and was made man." These are familiar words from the Nicene Creed. Christ became everything that a man is, except that He was without sin. Hippolytus had a better grasp of the duality of natures in Christ.
Similarly, the central feature of Tertullian's Christology was its understanding of the two natures of Christ. He called them "substances." The Word in his view has existed alongside the Father from all eternity, a distinct Person but one with the Father in essence. He became man since only as a man could He accomplish our salvation. He was born from the Virgin and as the Son of God needed no earthly father, but it was necessary for Him to derive his manhood (i.e. humanity) from an earthly source. Consequently He entered into the Virgin, and received His flesh from her, in a real birth — from her not simply through her. Christ's humanity was in every respect genuine and complete. He hungered, He thirsted, He wept, He died.
So side-by-side we see His Godhead and His manhood, divine spirit and human flesh. Mortality and immortality, strength and weakness. When we say that Christ suffered and died, however, the reference is to the human substance (nature): per Tertullian God does not suffer. However in my opinion Tertullian's view of God's impassibility in this regard does not seem consistent with the unity of the two natures in Christ proclaimed by Chalcedon — without confusion, separation or division. Tertullian posits that God does not suffer. Tertullian's approach has the merit of endowing Him with a full humanity without undermining the union of the human nature (flesh) with the divine Word (Logos). Novatian later modeled his ideas on those of Tertullian, declaring that Christ is both God and man, combining "both substances" in Himself. But he tended to hold the two natures so far apart that he has been accused of Nestorianism before Nestorius. Clearly, these thorny Christological issues were not to be settled by one man or group of men, but had to wait until the Chalcedonian formulation of 451 AD.
Irenaeus was important to early ecclesiology because he pulled together all the second-century ideas about the church in a sharpened attack against the Gnostics. Irenaeus regarded the Church as the new Israel, Christ's glorious body and the mother of all Christians. It is mystically endowed with powers without charge and bestows infinite graces. "Where the Church is," Irenaeus writes, "there is the Spirit of God."
Irenaeus regarded the Sacraments as external rites which Christians believe convey unseen graces. From the beginning baptism was the universally accepted rite of admission to the Church. Only those who have been "baptized in the Lord's name" may partake of the Eucharist (see the Didache 9, 5). Baptism was held to convey the forgiveness of sin, and for Irenaeus the seal of eternal life and our rebirth in God. In the early church the bread and wine were considered holy, spiritual food and drink. To partake of them is to partake of immortality, to partake of the divine nature. The bread is the flesh of Christ; the wine is the blood of Christ; to consume these brings the Christian to union with God.
Irenaeus teaches that the bread and wine really are the Lord's body and blood. In these we have the hope of resurrection to eternity, granted by the very words of Christ Himself whilst He was personally present here on earth.
As importantly, Irenaeus believed that the Church is the sole repository of truth. It has the apostolic writings, the apostolic oral tradition, and the apostolic faith. The church is "one holy, catholic and apostolic" because of the faith of the Apostles. This contrasts sharply with the various, and often contradictory, teachings of the Gnostics. The unbroken apostolic succession of Bishops in the great sees going back to the apostles themselves guarantees this faith is the same faith preached and proclaimed by these same apostles.
J.N.D. Kelly, Early Christian Doctrines, Revised Edition, 1978. Harper-Collins Publishers.