Recently, Barack Obama gained extra momentum because so many people were saying, "I think he will win." In fact, a lot of great political and religious movements, historically, became great because a Leader made promises and predictions beforehand... which later also happened to come true. When the leader's predictions did become true, the belief of the followers became that much more resolute. Their faith in the leader became stronger and more fanatical.
Watch TV interviews with any championship team in sports and they'll tell you about the moment their coach got them to believe in themselves (actually, something greater than themselves - themselves collectively, as a team). Of course, that's also the moment they increased their faith in their coach.
As I say, this happens all the time, mainly in successful movements and group endeavors. But Mark Goodacre (and most of critical scholarship along with him) doesn't think the followers of Jesus - who watched him fulfil one set of predictions in dying and rising again - would have published another set of their Lord's predictions (that I gather Mark may believe Jesus DID make) about the destruction of Jersualem before it had happened. To his great credit, Mark says the issue is NOT about whether Jesus made the predictions. Mark says,
[The issue] is about observing the literary function of successful prophecy in the narrative in which it appears. The prediction only gains traction because the reader is saying, "Hey, yes! I know what that's about!"Well, actually, Mark, History shows that a lot of predictions gain plenty of traction before they come to pass. As I've tried to show, examples from everyday life positively abound. :)
Such waves of anticipation uniformly lose all traction when hopes get destroyed. But the movement crescendos triumphantly when the Leader's predictions, published and repeated for months or years beforehand, are consummately fulfilled. And then the real bandwagon swell begins. At that point, even former detractors say, "Yes, look. It happened. Let's get behind this." (Been watching the news this past week?)
Clearly, therefore, I feel there's no reason to doubt whether Matthew and Mark could have written their versions of the Olivet Discourse decades before 70 AD. In fact, I think it's far more likely, given the increasing success of the christian movement afterwards, that they each spent a decade or two publishing and repeating the Lord's predictions about Jerusalem's destruction beforehand.
This isn't purely a matter of faith. This is logic with a different set of premises. :)
Update: Mark posted again about this issue on 12.23.08.