Historical events are based upon a testimonies that can't be definitively proven, so are you suggesting that the only justified position to take is to withhold belief about all of history?
When dealing with historical events, we have more to go on than just testimony. For example, if we have multiple accounts of a battle that was fought at a particular place, then we send an archaeological team to examine the area and they find weaponry or other leftovers from the battle, then we can say the accounts are reliable.
Likewise, certain major historical events would have had to happen for history to unfold the way they did. For example, we have multiple accounts of Caesar crossing the Rubicon. Had that not actually happened, the Roman civil war could not have happened as is otherwise documented, and very likely would not have happened at all.
There are times when we have a well known historian who has other works that have generally stood up to scrutiny well, and therefore can generally be called a reliable source. However even under those circumstances unless there are other accounts, we can't be certain how accurate the account is.
The bible doesn't even have that though. The gospel writers were all anonymous and likely copied sections from other gospels, and their works have been translated and edited so many times, we don't even know what the originals looked like. Paul is someone we do know wrote some of the New Testament texts, but even then a lot of things written in his name turned out to be forgeries.
Though I think the existence of God can be definitively proven through things like Aquinas' Five Ways,
How do you figure?
the issue is not about what can be definitively proven, but what can be sufficiently established beyond a reasonable doubt. Those who believe Christianity is true consider it to be a justified belief sufficiently established beyond a reasonable doubt and wouldn't not believe it to be true if we thought we weren't justified in believing it. While I find it difficult to believe that Jesus resurrected, I find it even more difficult to believe that Christianity survived and flourished without him having resurrected, but either way we're stuck believing something that is very difficult.
You don't need Jesus to have actually resurrected in order to have Christianity, you only need people to believe he did. Likewise a billion people believe Muhammad split the moon in half, and I'm sure they think their beliefs have been proven beyond a reasonable doubt. Just because a lot of people believe something doesn't mean that thing actually happened.
Of course we should strive to hold ourselves to the highest standards of evidence, but we often have to make choices about what to believe when we do not have all of the evidence that we would like.
If you are missing key pieces of information, you don't have to make a choice about what to believe. In fact, if the available evidence doesn't conclusively point one way or the other, the only justifiable position is to say "I don't know", withhold belief and search for more evidence.
If someone told you what they ate for breakfast today, then the highest standard of evidence would be to pump their stomach to see what they ate, yet we are nevertheless justified in believing many things without having the highest standard of evidence available. Naturally, things that are harder to believe require stronger evidence before we consider it to be sufficiently established, but it is unreasonable to refuse to believe anything until it has been definitively proven.
If someone claims they had pancakes for breakfast this morning, I'll accept it because it's a trivial claim, has no bearing on me, and is a fairly common occurrence. They may still be lying to me, but I really don't care because it's meaningless to me.
If you're talking about someone coming back from the dead, then an extraordinary amount of evidence would need to be presented. All the evidence we have shows it's impossible for people to come back from the dead. Therefore if someone did, it would overturn some major facts we know about how the world works.
In that case, we'd need something very compelling in order to justify belief. The writings of anonymous authors in an old holy book simply isn't enough to justify belief.