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Modern scholarship
The books in our New Testament were chosen by Catholic officials in the fourth century AD -- the modern list of twenty-seven books was first published in 367 AD. For 1,500 years after that, pretty much everyone thought the four gospels were accurate histories of Jesus' life.

So you can imagine how surprised modern believing scholars were when they first set out to write a comprehensive history of Jesus' life by combining the details in each of the four gospels. They made gospel parallels -- the text of Matthew, Mark, Luke and John in columns, side-by-side -- and discovered the gospels disagree with each other. Oops. 

And I don't mean the Gospels disagree in the sense Luke and Matthew give Jesus' genealogy and Mark doesn't. I mean they disagree in the sense that Luke and Matthew give genealogies with different people on them.

And the problem was way more than the genealogies. When you look closely at the gospels in parallel, you see that things that happen first in one gospel happen second in another. You see that Jesus says things one way in one gospel and He says them differently in another. Leading reasonable believers to see that the gospels are not 7-11 surveillance videos of Jesus' life.

Once the facts forced people to understand the gospels are not reliable histories, it was Katy-bar-the-door.

Pretty soon people noticed that the gospels describe different theologies. Mark's Jesus is a miracle worker who gives out his message in cryptic parables whose meaning he explains only to His inner circle. John's Jesus is self consciously transcendent. There's a long list -- my point here is to mention, not to blather details.

Other people noticed parts of the gospels looked suspiciously like pagan mythology and pagan philosophy. I'm sorry, but we don't have time to go into our Gospel's different theologies, or details about which one has which Pagan philosophy.

Many early Christianities
We do have time to look at how there were many early Christianities.

Jesus People
Here's a detail you maybe didn't pick up in Sunday school (they were busy, probably, doing other stuff) -- Jesus first followers were not Christians! -- they didn't believe Jesus was the sacrificed, resurrected savior. Yes, I do know this sounds nutty, but it's part of mainstream modern scholarship. Here's how scholars get there:

By the way

Paul letters weren't the only early pre-gospel Jesus-related (it's not accurate to say "Christian"!) writing. Other early Jesus-related writing included the collection of Jesus' sayings Matthew and Luke used to write their gospels -- it's called the Synoptics Sayings Source, or Q.

The Synoptics Sayings Source was written, and presumably passed around, during the generation or two after Jesus died and before the gospels were written. And, here's the thing, the Synoptics Sayings Source never mentions Jesus death and resurrection, or Jesus bringing salvation.

Harvard's Professor Helmut Koester describes >>

In the Synoptic Sayings Source, "chronological titles for Jesus are strikingly absent, [scholar-speak for 'Jesus is never called Christ'] nor is Jesus proclaimed as the one who rose from the dead and who will return in the future." [Helmut Koester, Introduction to the New Testament, volume 2, History and Literature of Early Christianity, 2d ed, pg153]

and

Synoptic Sayings Source is evidence "for the development of a Jesus tradition that had no relationship to the proclamation of the cross and resurrection of Jesus." [Helmut Koester, Introduction to the New Testament, , volume 2, History and Literature of Early Christianity, 2d ed, pg154]

The idea is, the earliest Jesus People told stories about Jesus, memorized things he said -- even wrote down His sayings that were important to them, and passed them around. But they didn't record anything about Jesus being the messiah, or anything about Jesus being resurrected, or anything about Jesus bringing salvation. Wow. Jesus' earliest followers weren't Christians!

The Synoptics Sayings Source isn't the only remnant the early pre-Christian Jesus people. In the orthodox tradition, there are also the Synoptic Apocalypse and the Collection of Parables.

Again, this is all mainstream scholarship. Tedious and pretty complicated mainstream scholarship; but widely written about and widely accepted. If you're of a mind to read about it, I suggest you start with Professor Koester's excellent book, chapter 10. [Helmut Koester, Introduction to the New Testament, volume 2, History and Literature of Early Christianity]
The Roman Christian tradition wasn't the only game going in the pre- New- Testament-gospel decades after Jesus. The gnostic Christians were around then too, and they used the Gospel of Thomas (don't look in your New Testament, the fourth century Catholic priests didn't put it there), another mid-first-century collection of Jesus sayings. And again, no mention of Jesus death or resurrection. Wow. Jesus' earliest followers weren't Christians!

In fact, get this, although gnostic Christianity survived for hundreds of years -- and in places like Egypt was older and stronger than orthodox Roman Christianity -- the gnostics never got around to having Jesus be the savior by way of his death and resurrection! For the gnostics, Jesus brought salvation by bringing wisdom, wisdom you could learn from another gnostic. No cross. No resurrection. Did your Sunday school class skip this too?

An early schism
The New Testament book of Acts describes the orthodox history's first Christian schism. It happened right after Jesus' death, likely in the 40s AD. In Jerusalem the first Christians thought Christianity was Jewish. You wanted to be Christian? -- first you had to convert to Judaism, to be circumcised and to follow Jewish ritual.

St. Paul, from the Hellenized city Tarsus, saw it different. You became Christian, you didn't get circumcised (whew!), you didn't have to follow Jewish ritual -- you didn't convert to Judaism.

The two sides argued. You read the New Testament, you get the feeling Paul and his pals won. That's because the New Testament is partly a product of the first Christian schism, written before the convert / don't convert question was finally settled, including gentile- permissiveness as a theme, spinning Paul's point of view. 

By the way

These different Christianities were often defined by beliefs about the "nature" of Jesus -- was he "consubstantial" with God, was he really human or really God, or really both, or what? That makes sense -- the first Christians had to fit the square peg of Greco-Roman Gods- and- divine sons into the round hole of strict Jewish monotheism. Three Gods that were really one God was a new idea -- something to argue about.

Other things weren't new, and people didn't argue about them. What the early Christians didn't split into new sects over were baptism, the Eucharist, the soul, eternal life, salvation, etc. -- exactly the things they inherited from the other ancient Pagan religions. Everyone knew how they worked. Why argue?

How we got our New Testament is a long story. Christian writers quoted Paul's letters in the first century. They must have existed by then. But no one mentions our Gospels until well into the second century. If they existed before then, there is no evidence at all that anyone knew about them.

By the way