Let's start with the New Testament, since Christians are nuttier about believing the Bible to be historically accurate than Jews.
Virgin birth. There are many possibilities here.
- It's a meme of the religions of the time, and could easily have been attached to the mythology around Jesus after the religion started taking off. This story added credibility to Christian claims, because it's something the people would expect of a deity.
- Mary, or Joseph, or the family, or the followers, LIED. Not as likely as #1 above, but possible. Getting people to believe it wouldn't be as hard as it would be today.
- Mistranslation. The first writers/transmitters never said this but it got translated this way. And Catholicism loves it that way so they perpetrated it.
- Completely made up.
- Something unusual happened that wasn't very impressive, so it was exaggerated to be worthy of mythical/miraculous status.
- Trickery. The disciples put wine into water barrels, or had a stash of bread and dead fish at the ready.
- Numerology. Any time miraculous numbers are mentioned in the Bible you have to suspect a total dissociation from reality due to possible magic numbers being used to make some point.
Healing. Really? We don't have to look further than examples of faith healing today to know that they could have been false then but here goes:
- Lies. Gotta convince the masses to convert, so some miracle stories are in order. Easy stories to make up. It's not like people in Italy or even Lebanon would have been able to verify something like that. How many people were named Lazarus? You would be hard pressed even in a well documented society to figure out which one was named.
- Fakes. Shills brought out to fool the crowds. How hard would it be to fake a withered hand? Blindness? Lameness?
- Spontaneous healing, due to the effect of faith on the mind of the believer, not intervention by a deity or a magical power. Or, the person is so swept up in the moment they have momentary improvement. Did anyone follow up on these people a year later? No, of course not.
- Actual sick people being made to look more healed than they are. The disciples support the lame person in such a way that they seem to be walking, or straighten out the "withered hand" by force.
- Confirmation bias. Would Jesus' followers really document the many times he was unsuccessful? (assuming any of it is historical)
If you're going to believe the miraculous claims of one group of bronze-age people without question, you have to believe all of them. I don't see Christians pointing to the miracles of other religions as evidence that miracles happen, only the ones from their own religion.