One of several lines of evidence for the resurrection is the violence the early witnesses willingly suffered. Christianity arose in a very hostile environment. Obviously, any community willing to kill God will not hesitate to kill His followers either. And they didn't. Christians were persecuted by both government and citizen alike, with all but one of the 12 Apostles eventually being killed for their faith (and John, the survivor to old age, didn't exactly breeze through life either). We must ask the question as to why, if 12 people conspired to lie, would they give up their lives for it? Or why would anyone else from their time be willing to die for the same thing?
Unbelievers often misunderstand/misrepresent this argument. Obviously, dying proves nothing. Countless people have died for numerous contradictory beliefs. People have died for Christianity, but people have also died for Islam. So martyrdom alone cannot establish the truth of a belief. But martyrdom alone is not being used to establish the truth of Christianity in this argument. Christians have been dying throughout history, right up to the present day, but this argument only applies to those in the first century. In other words, those who were in a position to know if their profession of faith was correct.
If I died for my faith, that would prove nothing. Yes, I am a Christian, and I would hope I would have the bravery to maintain that profession under torture, or threat of death. But if I died, that would not prove I was right. It would, however, prove I was sincere. Most people are not willing to die even for the truth. If you put a gun to someone's head and told them to say "2 + 2 = 5", they probably would, even though a gunshot is a relatively painless way to go. But if you start throwing people to dogs, or coating them in wax to set them alight, or stoning them, or crucifying them, suddenly there is motive even to lie. If people are not willing to die for what they think is true, why would they die for what they know isn't true?
This is what is unique about the first century Christians. Modern Christians die for beliefs they have received. Ancient Christians, however, died for what they actually saw. Obviously, this isn't the case for all of them. Some of them, maybe even most of them, never met Jesus. But did Peter? Tradition holds that Peter was eventually taken to Rome, where he requested to be crucified upside down, as he did not feel worthy to die in the same manner as Christ. Peter wasn't dying because someone told him Jesus rose from the dead, and he just happened to believe it. Peter was dying for the testimony that he had actually seen Jesus alive after His death. Furthermore, He was one of the alleged parties to the theft of Jesus' body. This was the prevailing excuse for the empty tomb in his day. Jesus didn't rise, the disciples just stole the body to make it look like He had.
Now, if any of that was valid, Peter would have known. He would have known he was party to some grand conspiracy to fake the resurrection (which would have involved a fisherman overwhelming two Roman centurions). At the very least, he would have known he never saw Jesus alive. His death may not prove the validity of his beliefs, but it does validate the sincerity with which he held them. Rather than knowing he was a liar, and confessing this under pressure, he had so much faith that he was telling the truth, he lay down his life for it. He, and 10 of his friends, all died for the same testimony. John, while tradition holds that he lived to old age, would surely have done the same, for he also endured much punishment for his faith. Meanwhile, if any Christian at all recanted their faith under pressure (keeping in mind no testimony extracted by torture is valid, for people will lie to avoid pain), the record of it has long since perished.
It is a fact of life: Liars make poor martyrs. People break under pressure, even the stubborn ones. When pain increases, when death is on the table, most people will say or do anything to get out of it, even tell a lie. The fact that so many people were willing to not only endure brutal torture, but rejoice that God considered them worthy to receive it, demonstrates sincerity. This forces us to ask the question: why did the Apostles believe they saw the risen Lord? There are many ridiculous answers to this. Maybe Jesus wasn't really dead, and just passed out on the cross. How He then managed to get up and walk around as if nothing had happened is anyone's guess. Or maybe Jesus had a twin brother... that His own mother didn't even know about. Maybe the Apostles were just hallucinating. Hallucinating the same thing, at the same time, along with a wide range of both disciples and sceptics. These are all genuine attempts to explain the sincerity of the Apostles. They genuinely believed they saw Jesus, but there could be any number of explanations as to why that don't involve Jesus actually rising. However, the simplest and most logical explanation is that they believed they saw Jesus, and were willing to die for it, because they did see Jesus. There is no need, and no room, for any alternative explanation.
And this is just one line of evidence. One simple fact that can fit into the acrostic poem "ALIVE". So Jesus rose. What does that mean for you? Good news! What it means is God's door is now open. Sin separates man from God, for what fellowship has light with darkness? When the light is switched on, the darkness disappears. When God is "switched on", the wicked die. And you are wicked. But for your sake, Christ was crucified. Christ took your punishment. Because He died, you don't have to. Because He rose, you can inherit eternal life. All it takes is faith, which the evidence shows is more than reasonable.