Assuming the apostles existed, were all (or mostly) martyred, etc. doesn’t thereby prove the accuracy of Christian beliefs.
American soldiers who volunteered and go off to fight in war, if tortured enough could surely be brought to say that they repent having come. However, very rare is it that the torturer would actually care to get such a statement out of them. More likey they would be more interested in getting tactical information, or simply in punishing the soldier.
Personally I would state that volunteer soldiers are very similar to the martyrs. They are a group of people going out and doing what they think has to be done for the greater good, even if it means that there is some chance that they might be captured, tortured, and executed.
But this doesn’t mean that their belief in the greatness of America (or whatever) is founded on anything real–nor that that should be a reliable argument for people to believe in America 2000 years later.
I would never go to war to defend the idea that the earth is round. But I would go to war to defend my home and my livelihood. That is a personal choice on my part that there are some things that are worth fighting and dying for. And none of that has to do with me being insane, believing in a cult, wanting to appear the noble martyr, nor any particular belief that my home and livelihood are particularly better than those of the people who might be attacking me.
So, I mean, it is easy to be afraid of death and extreme, gruesome pain. But just because you and most people you know might not be able to think of anything that would make such worth it to you, does not mean that those who can think of such things know something you don’t.