The manner of the hypostatic union then became an urgent problem. The Church of the fifth century was called upon to attempt a solution. Any reading of the Gospels compelled the recognition of divine and human elements in Christ; but speculative theology found it difficult to reconcile that fact with the equally important fact of the unity of person.
The theologians of the previous century had bequeathed little or no guidance. The fifth-century Christologians were pioneers in an unmapped region. Athanasius' great treatises on the incarnation are hardly more than eloquent defences of the true deity and true humanity of Christ. They contain little or no constructive Christology. Their theme is, _autòs enênthrópêsen_, _hína hêmeis theôpoiêthômen_. He maintains the fact, but does not deal with the "how." He uses the phrase "natural union" (_hénôsis physiké_), but does not attempt to define the mode of that union.
Apollinaris was, as far as we know, the first theologian to approach this subject. We may note in passing that, though he was bishop of Laodicea in Syria, Alexandria was his native place. His father was an Alexandrian, and he himself had been a friend of Athanasius. The fact of his connection with Alexandria deserves mention, because his doctrine reflects the ideas of the Alexandrian school of thought, not those of the Syrian. Apollinaris set himself to attack the heretical view that there were two "Sons"--one before all time, the divine Logos, and one after the incarnation, Jesus Christ. In doing so he felt constrained to formulate a theory of the union of natures. He started from the Platonic division of human nature into three parts, rational soul, animal soul, and body. He argued that in the statement "the Logos became flesh," "flesh" must mean animal soul and body. He urged in proof that it would be absurd to suppose the Logos conditioned by human reason; that rational soul was the seat of personality, and that if it were associated with the Logos, it would be impossible to avoid recognising "two Sons." He expressly asserted that the humanity of Christ was incomplete, contending that this very defect in the human nature made possible the unity of His person. According to Apollinaris, then, the union was a composition. The Logos superseded the human reason, and was thus united to body and animal soul.