Bruce Charlton's post "It seems that all actual religions are honest about what they themselves offer (but wrong about other religions)" makes a fascinating and important observation:
"I find it very striking - although I don't know of anybody else who does - that actually existing religions seem to be honest about what they offer their adherents."
I understand "actually existing religions" to mean religions that aren't just made up (like Scientology), but are based on some insight into the nature of reality. (And this includes philosophies like Stoicism or Platonism).
I would go farther and say that not only in what they promise, but in a broader sense, religions are constrained by their underlying nature. They cannot change open-endedly and retain spiritual power and if their adherents try to change them in a way that is incompatible with their underling nature, things will go wrong and the change will cease to be viable (though maybe not all at once). The exact details will be different depending on the religion and on the change.
In this post I want to think through this in relation to Christianity and cohesion. My motivation comes from thinking about the fact that there are many different denominations of Christianity and wondering if they ever will become one again and if not, can they find cohesion in some other way?
At this point, I do not think there ever will be a single denomination of Christianity again. Human beings are fallen, different from each other, and also the split between the denominations has gone on for so long and is involved with many other doctrinal, cultural, and historical issues.
But is this purely a result of human weakness or is it somehow part of the nature of Christianity? After thinking it over, I do believe that differences (though not acrimonious divisions) among Christians would always have occurred since freedom and individuality are inherently part of Christianity.
To begin with, consider the 11 Apostles after the Ascension of Jesus. Early on, we see that Peter was their leader. He spoke at Pentecost and is shown taking a leadership role in other parts of the Acts of the apostles. But from the descriptions given in the book of Acts, it appears that Peter's role was more that of "first among equals" rather than the ruler of the other Apostles. Christians reading about these events after they took place are familiar with this fact. However, one might have guessed that Jesus would have chosen one successor to be the unquestioned authority over all other Christians, just as Soloman was the king after David.
In addition, at some point, the Apostles went their separate ways, preaching in different places. It does not appear to be the case that they were commanded by another human being to go to any particular place; they went where they believed they should. Paul says in the letter to the Galatians (1:15-24):
"But when it pleased him, who separated me from my mother's womb, and called me by his grace, to reveal his Son in me, that I might preach him among the Gentiles, immediately I condescended not to flesh and blood. Neither went I to Jerusalem, to the apostles who were before me: but I went into Arabia, and again I returned to Damascus.
Then, after three years, I went to Jerusalem, to see Peter, and I tarried with him fifteen days. But other of the apostles I saw none, saving James the brother of the Lord. Now the things which I write to you, behold, before God, I lie not. Afterwards I came into the regions of Syria and Cilicia. And I was unknown by face to the churches of Judea, which were in Christ: but they had heard only: he, who persecuted us in times past, doth now preach the faith which once he impugned: and they glorified God in me."
In other words, Paul viewed his mission as in parallel with the other apostles; it was complementary to their ministry, but was not ruled over by them. If Christianty from the beginning was meant to be a religion fundamentally based on submission to external authority, Paul could never have written these sentences.
Also, what about the Ethiopian eunuch described in Acts 8:26-40? Philip meets the eunuch returning to Ethiopia and baptizes him, but after returning, the eunuch may have had no contact with any other Christians for a long period of time. Surely Philip knew that the eunuch would tell others of what he learned, but there is no mention in this passage about Philip forbidding the eunuch to teach anyone else about Christianity or telling him to submit to the authority of the apostles.
Also, the character of the teaching of Jesus was not based purely on submission to authority. Jesus taught both the Apostles and ordinary people in such a way that they would be able to understand and internalize the teachings for themselves. One the other hand, those among the Pharisees whom Jesus criticized probably did not teach the common people in this way. Their goal was order (as they envisioned it).
Yet, despite the lack of a single centralized authority which demanded submission, there was indeed cohesion among the Christians after the Ascension of Jesus. It was a variety of individuals all working towards a single goal. Therefore, cohesion among Christians can come about by some other means than unity under external authority and indeed did come about in just such a way at the very beginning.
Side Note: About a year ago, I had another idea related to this, which was that three of the biggest denominations among Christians were there from the beginning. We can associate Peter with the Roman Catholic church, based on the tradition that he was first bishop of Rome, Paul with the Protestants as he preached based on his own knowledge of the scriptures, and John with the Eastern Orthodox as he was traditionally bishop of Ephesus, the captial of the Roman province of Asia and was also more mystical than the other two.
Interestingly enough, in his story "A short tale of the Anti-Christ", Vladimir Soloviev includes Peter II as the representative of the Roman Catholics, the Elder John the representative of the Eastern Orthodox, and Professor Pauli as the representative of the Protestant churches. If two people have had this idea, then probably many more have as well, so there is certainly more thinking to be done along these lines.