One excellent evidence for the truth of Christianity is the fact that it was started by a bunch of martyrs. It must be confessed that the idea that all of the Apostles, minus John, were martyred may not necessarily be the case. There are conflicting traditions. However, it can be demonstrated that at least some of the Apostles were killed for their faith, and the rest were at least willing to die, and did indeed suffer great persecutions.
Many atheists misunderstand this argument. It has never been "they died for their beliefs, therefore their beliefs are true". This argument only works when applied specifically to certain members of the early Church. It would not work for followers of other religions, modern Christians, or even many of Christ's own contemporaries. In reality, the argument requires a very specific claim to be made: "This is what I saw".
You may well be familiar with the sarcastic phrase "who are you going to believe? This source, or your lying eyes?" It's becoming increasingly common as the fake news media loses its monopoly. The reason it exists is that although it is entirely possible to trick your "lying eyes", this only goes so far. You can trick people into thinking you're randomly shuffling a deck of cards, while in reality you know exactly which card you're holding on to. You might well think you see water running down a dry hill on a hot summer's day. It is quite possible to create amazing optical illusions, even making two people see a different colored dress. But the whole point about having senses is that they actually work! Deception can only go so far.
Most modern Christians accept the faith based on a certain amount of evidence. Most of us are either raised in the faith, argued into it through apologetics, or both. If we die for what we believe, we are dying for what we have received. But the Apostles were dying for what their "lying eyes" saw. They saw Jesus heal sick people. They saw Jesus banish demons. They saw Jesus walk on water. They saw Jesus feed crowds with a child's lunch box. They saw Jesus get taken by the Romans, nailed to a cross, and eventually stabbed in the side. They saw Jesus alive again after He died.
There is only so far you can take the "lying eyes" argument. In fact, the irony is, the further you take it, the less reliable it becomes. If our senses are that unreliable, how can we rely on them to tell us they're unreliable? The same senses that occasionally trick us allow us to expose that deception. When you know what you're looking for, you can watch a magic trick more closely. There's always that one jerk who wants to make a trick fail by looking for how it's done. You can test your optical illusions. You can ask your friend if they see the same thing you do. Human senses work, and they allow us to assess our reality. Little things like magic tricks, optical illusions, hallucinations etc. are the exceptions, not the rule.
The reason the Apostles' will to die for their testimony is strong evidence for its veracity is that it is very tough to explain any other way. You can't wave it off as "anyone can die for a lie", because the Apostles are the source of the supposed lie, but they are sincere, and a sincere lie is a contradiction. The Apostles were in a position to know if what they were saying was true, and their actions suggest they believed it is.
There are other attempts to explain why the Apostles believed what they did. Bart Ehrman does the wise thing in advising his students not to use any of them, because any semi-competent Christian can shred them in a heartbeat. These explanations range from group hallucinations (which don't happen, and would destroy our ability to pool our knowledge if they did) to Jesus having a twin brother (let's be honest, you'd think Mary would at least know about this...).
Ultimately, the absolute best explanation for the Apostles' belief that Jesus rose is that Jesus rose. He didn't swoon, He wasn't some mythical figure that managed to appear to 500 people at once, He wasn't an alien, He really is the Christ, the Son of the Living God, who died for our sins according to the Scriptures, and that He was buried, and that He rose again the third day according to the Scriptures, and that He was seen by many witnesses. Any other explanation is just plain bonkers.