Christendom was a Christian culture and civilization that, historically speaking, began with Constantine. It started to divide with the Great Schism between East and West in the 11th century. It divided again with the Protestant Reformation beginning in the 16th century. It further divided during the Enlightenment movement beginning in the 18th century. Christendom has so divided that the word is almost archaic now.
Where does Christian unity come together? Where does it converge? Although Christ is the head of the church, where is the unity on earth after his ascension?
Since the early centuries of the church, the elders, called bishops, united in regional hierarchies that maintained mutual respect. When heretics threatened to divide the church, it was the bishops that met together to define the line between orthodoxy and heresy. The bishops united formed the unity of the church on earth.
In time the bishop of Rome asserted authority over all the other bishops. As Rome was the seat of the empire, so it should be the seat of the church. As the apostles Peter and Paul had been martyred in Rome, so there must be a divine seal on that location. The papacy was the convergence point of Christian unity, at least in the West.
The papacy has a simplicity that makes it a strong point of unity. However, it means that a pope may compromise the whole church. If he decides to sell indulgences, then the church sells indulgences and is compromised. That was the last straw for Luther.
Luther wanted to reform the church, which many agreed needed reform. But it was not to be and the Reformation turned into a schism. What did the Reformers propose as the convergence point of Christian unity? The Holy Bible. With the advent of the printing press and the translation of the Bible into the common languages of Europe, the Bible was newly available to the whole church.
Although the Bible has the advantage that it doesn’t change, it doesn’t do anything by itself either. What is called sola scriptura presumes some agreement on what the scriptures mean. That turns out to be harder than the Reformers thought. So the Reformation led to the formation of hundreds of groups, each of which revered the Bible but disagreed on some point of doctrine.
Where does that leave Christians today? Largely disunited, and unable to work together for common purposes. That is a point of weakness. The ecumenical movement attempted institutional unity but missed the unity of heart and mind. Progress has been made to at least recognize one another but common action is rare.
What can be done to achieve Christian unity now? The best that can be done is to foster specific projects that Christians can agree on. Aid to the persecuted. Assistance to the poor and marginalized. Reaffirmation of Christian morals. And prayer for unity.