I'm too lazy to formulate it myself, so I'm just going to quote this guy (David Sklansky) who wrote this on another forum:
What I am about to tell you is fact. If a person tells you something that seems hard to believe, there is a calculation you can perform if you previously held two opinions. This calculation is totally valid even if the subject matter is God. It works like this.
First come up with the probability in your mind that you would have given this piece of information before you were told anything. If the person told you they flipped ten fair coins and got ten heads it would be 1/1024. If they told you that their 51 year old sister was pregnant it might be 1/15,000. Those would be the chances BEFORE you heard the story. Now that you heard the story you ADJUST based on your opinion that the speaker would lie or be mistaken. Suppose you think that there is a 1/100 chance this person would lie or get things wrong. In other words you think this pers is 99% reliable. The problem is that if his contention outlandish he is still probably wrong. If he is asked about 15,001 different 51 year old females he will tell you the pregnant one is pregnant but he will say the same of 150 who are not.
His reliability allow you to significantly decrease your level of disbelief but not to the point where you can acrtually believe him.
Of course, religious people will assume their prophet is 100% reliable, but it might be an interesting concept for historicians.