Thanks in no small part to the efforts of Athanasius, the Arians were cast out of the church, and the orthodox, biblical view of the deity of Christ prevailed. After decades of exegetical and theological arguments against Arianism, a church council was called in AD 381 at Constantinople. There, church leaders reaffirmed the decisions of the Council of Nicea on the deity of Christ, and the fact that the Holy Spirit is also truly God was also clarified. The end result was the Niceno-Constantinopolitan Creed, which Christians around the world confess today, usually referring to it as the Nicene Creed.
Christological debate and discussion continued, but the humanity of Christ became the new center of theological controversy. The important players in the initial phase of this argument were Nestorious, the bishop of Constantinople, and Cyril, the bishop of Alexandria. Nestorious, driven by a concern to preserve the full deity of Christ, began speaking of Jesus as two persons, one human and the other divine. This view has two major problems. First, it ultimately ends up presenting a Savior with a divided personality—a being divided against Himself. The second issue is that it gives us a Redeemer who did not truly become incarnate. God the Son did not truly unite Himself to humanity, which means that God the Son cannot offer a satisfactory atonement. That which is of infinite worth, the divine person, cannot give infinite worth to an atonement made according to finite humanity if the human nature and divine nature are not truly united in one person. Cyril argued that there had to be a true union of humanity and deity in Christ that preserved the distinctive attributes of each nature while making the divine person able to save human beings. An ordinary human being is finite, and his sacrifice cannot do anything for anyone other than himself. But a divine person with a true human nature is of infinite worth, so He can save the world. Cyril's position was affirmed at the Council of Ephesus in 431.
Yet the christological debates were not over. A variety of other heresies had to be addressed, including monophysitism, which held that the divine nature swallowed up the humanity of Jesus. That finally produces a Jesus who cannot save us because He is not fully human. In 451, the Council of Chalcedon reaffirmed the decisions at Ephesus and condemned monophysitism and other heresies. That has left us with the orthodox belief that says Christ is one person with two natures, each nature retaining its particular attributes.
The christological debates of the early church can be hard to keep track of, but there is one important conclusion from them that we can easily remember, namely, that Jesus' humanity is as important to our salvation as His deity. Because He was and remains truly human as well as truly divine, Jesus atoned perfectly for our sin and helps us in our temptation. He can save us and sanctify us because He is one of us without sacrificing any of His deity.