Owen,
if your first post is the real MAIN reason why you are an Atheist I am pretty worried about you. If you think that just because of a 40 year gap between a document and a historical event the records are utterly and completely unreliable, then how do you ever know anything in history? There are still people writing history books on World War II. I hope you do not read them. let's see, 40 years ago was 1970... So, if you ever meet a Vietnam war veteran, do not even think about believing a single word he says about it. It is all lies, and he is suffering from war trauma anyways.
If you don't get it on tape or video from then, do not believe it. It is all secondary stuff, and has been corrupted. You know, gossip and all that. Yeah, your parent's childhood: Lies I tell you, nothing but lies. Do they even have any evidence that they really lived where they claimed 40 years ago? Do you have pictures, super 8 films? That house probably doesn't even stand anymore. You know, they are actually government agents, and you were placed in their care right after they stole you from your real mother, who was an hair to the throne of Russia, but the CIA was afraid you'd be the next Russian emperor. It's all made up, but since it was so long ago all the people seem to remember your "arents."
Seriously man, if a 40 year gap is your main reason for being an Atheist, you are skating on very thin ice. Let me give you the main reason for that:
You do not have 1 single piece of evidence that there was a thing altered in this time. You are relying entirely on speculation in this argument, and you do not have anything in form of hard evidence to prove that even a word was changed in any of Jesus' speeches in that time. So your entire world view rests on a completely unsupported assertion that something WAS changed. Actually, an assertion that almost EVERYTHING was changed. I would go so far to say that the real reason you think something was changed is because you want to be an Atheist, and not the other way around. No credibly historian would throw away a source just because of a 40 year gap.
Ok, let's go one further:
God can neither be proved to exist, or be proved to not exist. Therefore, he is out of discussion.
That, also, is an unsupported claim. How do you know that God cannot be proven to exist?
Are you going to say, that only things verifiable by science can be proven to be true? (That's called positivism)
Well, the problem with that view is that you cannot prove THAT statement by science. What kind of experiment would you do for that?
I would argue that you can very well prove that God exists. There are a good number of proofs, such as the Kalam Cosmological argument. I am not saying that it does completely prove his existence, but it COULD.
Or, if you could establish by historical evidence that Jesus really was raised from the dead, I would take that to be strong evidence that God exists. I am not saying this is a waterproof argument, but it COULD be.
How else would you like it proven? There is a purely logical argument as well. The shortened form is that if it is possible that God exists, than he necessarily exists (There are about 20 steps in between, check Alvin Plantinga's Ontological Argument if you are interested)
Then, of course, we could argue that if Christianity IS true (which is possible) than all humans have a "sense of the divine" that if it operates correctly lets them see that God exists. They would know that he exists as a proper basic belief, as a result of that "sensory organ" of the "divine sense." That would be proof.
Another thing we could do is check into near death experiences, and see what that data shows. That could be close to a proof.
Anyways, I hope this gives some room for thought.