tyu: If God wrote on the moon with big letters: "The God of the bible exist", i would become a believer. I would daresay that is a simpler thing to do than killing your own son.
On the subject, consider Muhammed. A billion people believe he was the last prophet of God, i am quite sure we can both agree that is not the case. The kind of evidence muslims will cite is many: Muhammed has superior scientific knowledge (all of which is subject to interpretation, and has only been "correctly" interpreted after the scientific discovery was made independently), muhammed is mentioned in biblical prophecies (but allways post-hoc), muhammed talked with God (...but we only has his own words for it, and his immediate followers. Even if he really heard a voice it would, as i am sure you will agree, NOT be evidence in itself) and muhammed could do miracles (widely described and believed, but never objectively verified).
Most of the problems with believing in Muhammed comes down to the fact that the story of muhammed was only written down years after his death, when he had allready become legendary, and it was not independently written down by unbiased sources.
Lets take the miracles - Muhammed is said to have flown on a horse-like creature between two cities during a night. Ofcourse there are no objective witnesses to this, only the second-hand testemony written down years afterwards. Does such a story make it more or less credible Muhammed was a prophet? What if he was said to have flown on a horse every night?
At the end of the day, if i claim i had a flying horse, it just happened i only flew on it once and i had no independent confirmation, I think it would make you quite sceptical with respect to my trustworthiness in other matters. it should. It should make you wonder why i didnt choose to fly over central park and get a lot of eye-witness testemony!
If we return to jesus, the stories of jesus is only written down 40 years after his death. and the supernatural elements of the story of jesus is only written down by his followers. It cant even be confirmed that eye-witnesses to his miracles ever wrote down what they saw!