I know, it sounds like a science fiction story, doesn't it? "Exegesis" is a very cool word in its own right, but when "impulse" enters the game you know the result is going to involve lots of polished metal and/or advanced biotech.
Sorry to disappoint. This is actually going to be a discussion of scripture.
The word "exegesis" originally comes from a Greek word meaning "to lead out" - to garner meaning from a text based on the words within it. That's a fair enough pursuit. It covers pretty much the whole range of Bible commentary - technically, any study of the Bible that doesn't make reference to contextual historical information is exegetical.
I was actually quite shocked to read this definition. For some time now, I've understood the word to have a subtly different meaning. Exegesis, in many contexts, means "filling in the gaps by making shit up". It's in this sense that I'll discuss the word.
The prevalence of exegesis
You don't have to go too far to see that exegesis is not a rare phenomenon. Your average nativity play will have a whole host of details that aren't in the original text - the three kings, for example, as opposed to an unspecified number of wise men.
A more fertile field for this activity is creationism. From a scant two chapters of Genesis, creationists have devised a complete history of events surrounding the Earth's origins. It's got everything: huge sheets of water inexplicably falling from low orbit, a fascinating "hydrological sorting" effect to explain why trilobites always appear lower than turtles in the fossil record, holes in the Earth's crust for the water to hide in afterwards, etc. Some versions even have complex relativistic effects to explain how we can see 10,000,000,000-year-old starlight in a 6,000-year-old universe.
All these variants have two things in common. Firstly, they're completely implausible. To pick on one example, hydrological sorting can't account for the different radiological signatures of different layers, because lots of swirling water is too crude a tool to distinguish between isotopes.
Secondly, they're nowhere to be found in the Bible. Genesis doesn't say anything about fossils or plate tectonics or relativity. These ideas are exegesis - people see that Genesis doesn't appear to match up with reality, so they make shit up to fill the gaps.
How far back?
Exegesis is also not a recent phenomenon - even as early as the second century AD, people were adding little "finishing touches" to scripture. For example, in the Eastern Orthodox Church, it's strongly believed that Jesus was born in a cave. This is not mentioned anywhere in the scriptures, although it does show up in a couple of Gnostic gospels and in the beliefs of sundry contemporaneous religions about their messiahs.
So how far does the rot go? How can we tell?
One easy way to detect exegesis is what you might call a "comparative biology" of stories. For this I'll call on the writings of a blogrollee of mine, who discusses his deconversion story here. Come back when you've read it, OK?
As this story indicates, a major feature of exegesis is that it gives different results every time you do it. After all, if the meaning of your source was obvious then you wouldn't need to make shit up to fill in the gaps. So all we need to look for is completely different versions of the same story, and we'll know that one or both of the authors is happily exegesising.
A tale of two gospels
Enter Matthew and Luke. It's long been known that these two gospels share a lot of material with Mark and with each other, often word-for-word. The current best guess at which came first is known as the Synoptic Hypothesis. It's generally believed that Mark was first on the scene, and that Matthew and Luke had copies of his work handy as they wrote.
The reason for this is simple. Where Mark tells a simple story or leaves a gap, Matthew and Luke tend to elaborate - and they do so in completely different ways. Consider, for example, the Nativity story. Take the version in Matthew (Matt 1:18-2:23) and the version in Luke (Luke 2:1-40), and compare them. You will find precisely five points of overlap: the names of Joseph, Mary and Jesus, and the towns of Bethlehem and Nazareth.
Everything else, and I mean everything is completely different. Herod and the slaughter of the innocents are mentioned only in Matthew. Quirinius' census is mentioned only in Luke. Matthew talks of magi. Luke talks of shepherds. Matthew says the family fled to Egypt. Luke says that the family wandered over to Jerusalem. And will you just look at the two different lineages given for Jesus...
With sufficient rhetorical wriggling (exegesis!) it's almost possible to construct a single story that covers all the options. But, to be quite blunt, why the heck would you want to? There's so little overlap between these stories that they might as well be about different people.
To me, the reason for this lack of overlap is fairly simple. The authors of Matthew and Luke would have known that Jesus' parents were called Joseph and Mary. They would have known that Jesus came from Nazareth. And they'd have noticed a prophecy in Isaiah suggesting that the Messiah would have been born in Bethlehem. They took these facts and... exegesised. Matthew used Herod as his bogeyman to drive Joseph and Mary out of Bethlehem, whilst Luke took a completely different but believably bureaucratic option by attempting to link the birth to a Roman census.
Taking this as our working hypothesis, it instantly becomes clear why the historical census of Quirinius appears to have happened well before Jesus' birth, and why no contemporary author mentions such a barbarous act as the slaughter of innocents (despite trumpeting a range of Herod's infamies). It's because Matthew and Luke were making shit up.
How far back? redux
You'll notice that, up until now, I've been treating Mark as a reliable source, and only expressing skepticism about Matthew and Luke. There's a very simple reason for this: we have no earlier Gospels, so we have no way of knowing which aspects of Mark were historical and which were innovation.
Until very recently, I wasn't really bothered by this. Probably a few things in Mark were exaggerated a bit, but I saw no reason to disagree with the core of the story. Now, though, my feelings are different.
I've seen how fast exegesis can proceed, even in this modern information age where debate and criticism thrive. I've seen that exegesis was at least as powerful in ancient times - by most estimates, Matthew and Luke were written no more than a decade or so later than Mark. I've started to explore the motivations for these differences, in particular the three-way struggle between Romans, Jews and Christians that inspired much of the Bible's antisemitism. And I'm troubled.
As far as we can tell, Jesus died no later than 40AD. As far as we can tell, Mark was written no earlier than 65AD. That's at least 25 years gap, double the distance between Mark and the other synoptic Gospels.
Given that much time, and that much room for exegesis, how do we know that Mark wasn't... making shit up?