I suppose this is tangiential to the many 'Jesus Myth Theory' discussions that have popped up recently, but for me it is central.
Most religious figures (say, Zeus or God) have never existed, so accounts of them are easy to dismiss. Sources on figures that did exist, or may have existed (for instance Jesus), are potentially more difficult to evaluate. It has always struck me, though, that religious sources such as the Bible must automatically be discounted as historical sources of information.
Consider the following points:
* Religious sources are necessarily based on a false premise (most obviously the existence of God), which will automatically influence the interpretation and presentation of 'facts'. For example: Christian sources will construe Jesus as a prophet, or the son of God. Religious sources by their very nature propagate a worldview distinct from 'what really happened'.
* Some information in religious sources may be accurate - I'm not suggesting it's all made up - but the only way we could accept this information (or attempt some sort of interpretation) is if it was independently corroborated, i.e. supported by a non-religious historical document. In which case we believe the non-religious document. If there were only the religious document, we would be justified in dismissing it because, as already noted, the information it contains is at least potentially based on false premises, and may be a result of an accordingly skewed viewpoint.
Continuing with the example of Jesus and the Bible, if we dismiss all religious sources, we know very little about the 'real Jesus'. Although this is significant in itself, the real point is of course that the 'real Jesus' is both irrelevant to cultural history and of little interest in general. Heated debates about 'real Jesus' are thus somewhat pointless (did he exist? who cares?).
The real Jesus is not the one we need to worry about - he is an insignificant figure. The problem is the 'fake Jesus', as constructed by Christianity, who certainly didn't exist 'in real life', but whose influence on intellectual and moral standards continues to be very real...