Christianity is what I would call a historical religion, meaning that in order to be a Christian one has to accept certain claims about historical events. This is in contrast to a religion like Buddhism, where it’s followers would have no trouble with the central doctrine regardless of whether or not the Buddha was actually a historical person – the religion is not overly predicated upon the events of his life. The same could not be said for Christianity however; if one were to deny the resurrection of Jesus, and his ascension to heaven one could not be a Christian (at least not in the traditional sense).
There are many problems however with the historical claims of Christianity. Historians cannot establish with a degree of 100% certainty what happened in the past, they merely establish what probably happened based upon the sources that they have available to them. This is where Hume’s old argument against miracles comes in; a miracle is by (generous) definition extremely improbable – if people rose from the dead and walked on water all the time, there wouldn’t be anything particularly special about it, it’s the fact that it’s improbable that makes a miracle a miracle. How can a historian arrive at the conclusion that something which is by definition the least probable thing that could have occurred in a given scenario was the most likely thing to have happened in that scenario? Miracles have no historical worth even if they did actually occur – a historian could still not consider them to be the most likely explanation.
Ancient texts are full of miraculous claims, not just specifically Christian ones. This presents a further problem for the Christian making historical claims about miracles; if you are claiming that the miracles attributed to Jesus have historical worth what differentiates those claims from other non-Christian miracle claims? If you are willing to accept that Jesus rose from the dead, why is it that you would be unwilling to accept stories of yogis being able to levitate, or Muhammad riding a flying horse?
The only reason that Christians are willing to accept the miracles of Jesus and no one else is because they happen to be Christian. This is outright bias. Are we really supposed to believe that the only true documented cases of miracles are the ones that happen to be attributed to the leader of the faith you adopted or were born into? If you happened to be born into a Muslim family and culture you would be likely to argue that the miracles attributed to Muhammed were the only true miracles and that Jesus’ miracles didn’t happen. The only reason Christians believe in miracles is because it is necessary for their faith. An agnostic or atheist has no reason to give any more credibility to the miracles of Jesus than a Christian would give to the miracles of Muhammed.
Miracles have no historical worth, and the only reason one would claim that they do is because of a bias towards the religion they were brought up in, or adopted later in life.