Anyway, based on these studies, my feeling is that the prominence or legitimacy of alternative early Christianities is sometimes over exaggerated. I've often heard it presented as if groups like the various types of Gnostics or their collections of books were merely alternate but equally legitimate interpretations of the Jesus tradition - as if the difference between them and what became orthodox Christianity was no more significant than the difference between say, Methodists and Baptists.
In reality however, many of these sects were not just alternate Christianities, but really radically different religious belief systems that merely borrowed certain Christian texts and terminology and combined them with other mystical philosophies or religions. For instance, when I read the Nag Hammadi texts I am struck by their utter dissimilarity from anything you find in the Hebrew scriptures or the canonical New Testament writings. The closest analogy I can find is the difference between Mormonism and mainstream Christianity these days (though even that's not perfect since even the Mormons are probably closer to mainstream Christianity than most of the Gnostics were). Maybe an even better analogy would be some of the New Age-y cults out there that try to paint Jesus as some type of Higher Consciousness guru or swami, but with almost zero connection to the Jesus of history.
The other point to remember is that many of these early sects are much later developments. While all of the canonical gospels and the letters of Paul, John, and Peter can easily be shown to have been written in the first century, and usually within a generation or two of Jesus' ministry; most of the Gnostic gospels and other non-canonical texts cannot be shown to be any earlier than the mid to late second century (i.e. over 150 after Jesus) and most from he third or fourth century. It is for this reason primarily - i.e. the historical proximity to Jesus and the first apostles - that the canonical writings were preferred by the early church and were listed as such by many early Christian writers.
Of course, groups like the Gnostics, whose theology was so far different from orthodox Christianity as to be a different religion entirely, are somewhat different than early Christian groups like the Arians, Nestorians, or Copts (to name just a few) who were not outside of the mainstream of the Christian church, but who had differing ways of understanding the divinity and humanity of Christ. While I wouldn't say that these differences are unimportant, I would say that the disagreements between these groups were more akin to the denominational differences we have today, than they were to the radical differences between gnostics and Christians.
So why did the Nicene view triumph? I'm not entirely sure. There is the traditional answer, which is that it is the correct view, so the Holy Spirit directed the church to accept it.
Then there is the cynical, DaVinci code type answer, which is that political pressures by Constantine and powerful bishops conspired to suppress alternative views. While there is some truth to the fact that political interests had some influence over the process (especially in regards to the Emperor's desire for uniformity among Christians), this answer also conveniently ignores the facts that 1) Constantine himself preferred Arianism, and 2) that there really weren't any such thing as "politically powerful" bishops at the time of Nicaea, as Christianity had only recently gone from being a persecuted minority religion to being at least permitted (though not yet officially endorsed) by the Edict of Milan.
Personally I take a different view than either of these, which is simply that this is how the church decided things back then, through these sorts of ecumenical councils, and this is what they decided. There was a lot of argument and disagreement but in the end they reached a statement that most, if not all, could agree to. In fact, much of what was agreed on at Nicea (and at later councils like Chalcedon) was a kind of both/and compromise position that did its best to reconcile varying opinions. Since almost all currently existing churches today are descended from those who accepted the creed formulated at Nicaea, that statement is still held in high regard today. Though I suppose if someone wanted to make an argument that the Arians or Nestorians or whomever actually had a better way of looking at the dual nature of Christ than Nicene Creed does, and suggest that we ought to adopt that instead, they could certainly attempt to do so.
Indeed, that's my approach for all of these early "Christianities". I don't just engage them just on the historical level, trying to figure out who believed what, when. I also consider them as "live options" for my own belief. In other words, the question for me is not just whether Gnosticism was or was not an authentic expression of Jesus' teachings, the question is also whether or not Gnosticism is a persuasive view of the world to me. Would I want to be a Gnostic? Do I agree with their beliefs? Likewise, with other Christological views, do I think any of them are more right than the one expressed by the Nicene Creed? Nicaea isn't unquestionably right just because it's what became dominant, but it's not necessarily wrong for that reason either. As much as possible I want to evaluate each of these theologies and worldviews on their own terms and see which make the most sense to me. Speaking personally, when I do that, more often than not I do find myself landing on the "orthodox" option in the end. But I'm also glad that I've arrived there by my own effort and not just because I felt compelled to blindly accept some creed (nor compelled to disbelieve it simply because I can see the historical/political factors that influenced it).
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