Many conservative Christians say they want to return to pure, primitive New Testament Christianity. In the United States this is most often heard from Protestant fundamentalists. But many Catholics and Orthodox Christians also believe that their version of Christianity is the original one.
However, if you want to see what *real* early Christianity was like, you have to look at ancient sources. A fascinating one is the
Odes of Solomon, a collection of first century Palestinian hymns in Syriac (Aramaic). But for a more synthetic scholarly approach, Walter Bauer's book is one of the best. It is important to remember that "orthodoxy" is defined differently by different groups. Whatever your group believes is orthodox; whatever other groups believe is heresy. For many centuries, "orthodox" in the West has meant "Roman Catholic." But well before this version of orthodoxy existed, and for many centuries after it began, there were other Christianities.
Jewish Christianity and Gnosticism probably have the claim to being the oldest forms of Christianity. Marcionism, starting in the second century CE, quickly became the most widespread form of Christianity, and it remained so for centuries. Local varieties, such as that of the Syriac-speaking Bardaisan in Edessa, also sprang up. Eventually, though, the local variety of Christianity that sprang up in Rome managed to spread most effectively. Initially this was due to the natural organizational skills that characterized Roman culture. After Christianity was legalized in the Empire, Roman Christianity could avail itself of the resources of the state to aid its spread. (Even so, this only worked within the Empire. Outside of it, East Syriac Christianity was even bigger than Roman Catholicism for several centuries. See
The Church of the East: A Concise History.)
Bauer takes us on a long guided tour of early varieties of Christianity. He begins in Edessa and Egypt, "so as to obtain a glimpse into the emergence and the original condition of Christianity in regions other than those that the New Testament depicts as affected by this religion" (p. xxii). He then surveys the development of Christianity in other areas around the Mediterranean world. In each case he discusses what can be gleaned from ancient documents about the progress of Christianity in the local milieu, and how "orthodoxy" eventually became prominent.
Bauer's last chapter summarizes the advantages Roman Christianity was able to make use of as it pressed itself onto populations around the Mediterranean. He has telling statements, such as, "The form of Christian belief and life which was successful was that supported by the strongest organization..." (p. 231). With relation to the original Jewish Christianity in Palestine: "Thus, if one may be allowed to speak rather pointedly, the apostle Paul was the only heresiarch known to the apostolic age--the only one who was so considered in that period, at least from one particular perspective" (p. 236). However, the Jewish Christians--called "Judaists" by Bauer--were too inflexible in dealing with would-be gentile converts, while Paul was quite adaptive. "Thus the Judaists become an instructive example of how even one who preserves the old position can become a 'heretic' if the development moves sufficiently far beyond him" (p. 236).
Much has been written on this topic since Bauer's time, but his book remains a classic in the area. If you have a serious interest in the early Christianity, you owe it to yourself to get Bauer's perspective.