The two obstacles are related. If people are not allowed to understand that the narrative of John is as much a matter of experienced fact as that of Julius Caesar, and that both must be taken prima facie as eyewitness material, there is no matter of fact to be considered, only belief.
The point with Christian belief is rather different. It is that there are reliable, credible historical documents that make an incredible claim, namely that a man cured leprosy, insanity and blindness by command, raised the dead two or three times, calmed a sea storm, twice fed thousands from food barely sufficient for one person, and, having been very publicly and demonstrably killed, was soon after alive again. Obviously the point is whether you believe this account or not. That is why faith is accounted a virtue for Christians; NOT because you are supposed to believe IN DEFAULT of evidence or INDEPENDENTLY of any evidence.