The main assertion here is that since the gospels were written with the intent of converting people, the authors must have considered historical accuracy secondary, if not totally irrelevant. Of course, there is a kernel of truth here: people often have ulterior motives. We should certainly look closer to see if their religious motivation caused the gospel authors to distort history, but we shouldn’t use this as an excuse to just dismiss the gospels out of hand. This would entirely obviate the job of the historian, which is to sift through documents and determine what’s historically accurate and what’s not. If the presence of motives allowed us to dismiss a document without any further analysis, there would be nothing for the historian to do. But obviously this is ridiculous; this objection amounts to an all or nothing scenario where a document is either pure history or pure fiction.
The reason this is ridiculous is that all ancient histories were written with some ideological motive, be it religious or political. Indeed, it’s impossible to write a purely unbiased history. Moreover if what the gospels report on actually happened, we should expect the authors to go to great extremes to tell others about it with great zeal. In other words, we should expect them to do exactly what they did.
Another problem with this assertion is that there were many eyewitnesses of Jesus’ ministry, some of whom would have survived at least through the first decade of the second century. And since Christianity was controversial, these people would be considered to be able to speak authoritatively about Jesus and would be sought out to determine whether certain things about him were true or not. As F. F. Bruce writes:
And it was not only friendly eyewitnesses that the early preachers had to reckon with; there were others less well disposed who were also conversant with the main facts of the ministry and death of Jesus. The disciples could not afford to risk inaccuracies (not to speak of wilful manipulation of the facts), which would at once be exposed by those who would be only too glad to do so. On the contrary, one of the strong points in the original apostolic preaching is the confident appeal to the knowledge of the hearers; they not only said, ‘We are witnesses of these things,’ but also, ‘As you yourselves also know’ (Acts 2:22 [also 26:26]). Had there been any tendency to depart from the facts in any material respect, the possible presence of hostile witnesses in the audience would have served as a further corrective.
The fact that Christianity was centrally located in Jerusalem where many of the events took place makes the number of eyewitnesses here absolutely huge. This makes it very difficult to claim that gospels could be written which were not historically accurate but which would then go unchallenged by the hostile eyewitnesses (who wanted to destroy Christianity), not to mention the Christian eyewitnesses who were being tortured and killed for their belief and therefore had a stake in whether the gospels were accurate or not.
Moreover, there is very strong historical evidence demonstrating that the gospel authors were sincerely trying to record historical events accurately; such things as harmful details -- things that would not help, and would certainly hurt, attempts to convince people in first century Palestine that Christianity was true. For example, in all four gospels, women are the first people to find Jesus’ tomb empty. What is amazing about this is that women were believed to be hopeless gossips in first century Palestine and, as such, their testimony was considered completely worthless. Given this social and cultural attitude, what possible motive could the gospel authors have had for including this, other than that’s what actually happened, and they felt obliged to be truthful about it?
Other examples of this would include Peter’s denying that he even knew Jesus three times, recorded in all four gospels (Matthew 26:69-75; Mark 14:66-72; Luke 22:54-62; John 18:15-18, 25-27). Peter was one of the main evangelists, the last thing the Christians would want people to think is that he wasn’t trustworthy. This is true of all the followers of Jesus; they’re represented in the gospels as being cowards who just weren’t that bright. They were constantly misunderstanding Jesus, failing to trust him, and following their own selfish motives rather than God. These were the same people who were the leaders of the early church and trying to convince people that Christianity was true. They would never have disparaged their own characters like this unless these things actually happened and they felt obliged to be truthful about it.
Another type of unusual detail we find in the gospels are those that would have been, for some reason or another, difficult for people to understand. Gregory Boyd writes in Jesus Under Siege:
For example, Jesus’ parable about the farmer sowing seed which falls on rocky, shallow, and good soil, only makes sense in a Palestinian environment where seed was sowed before the ground was plowed (Mark 4:1-8 and Luke 8:5-8). Elsewhere throughout the Roman Empire the practice was to plow the ground first. Such accurate detail suggests that the teachings of Jesus were passed on in their original form, even in contexts in which the form of His teaching wouldn’t have made immediate sense. ... In just the same way, there is absolutely no discernible motive -- aside from the motive to "tell it like it really happened" -- for why the Gospels include the unusual detail that Jesus, while dying on the Cross, cried out, "My God, My God, why have You forsaken Me?" If the Gospel writers’ driving purpose was to portray Christ as the Messiah ("anointed one") and as the Son of God, this was the last thing in the world they would ever want to include in their narrative. They certainly wouldn’t make it up on their own!
But this is also quite common in the gospels: events that have enormous theological significance are detailed but not explained (John 20:17, for example). If the gospel authors were making up stories about Jesus they wouldn’t leave such important threads hanging. But if they were trying to record accurate history first and theological reflection second, this is exactly what we should expect.
Update (15 Feb 2010): See also part 1, part 2, part 4, and part 5.