I believe Christianity to be true because under pressure, under threat of death, people refused to give up their faith.
There is this thing called Catholic tradition. That's putting it nicely - what we should really call it is Catholic invention.
See, the Gospel of John was written by - yep, you guessed it - John Doe. We don't know who wrote it or any of the gospels. If you actually read them you will see that they are anonymous; there is no actual claim being made of authorship anywhere. There is also no historical evidence to show who wrote them so what we have is Catholic
tradition invention stepping in to fill in the blank. To be fair, I suppose it could be said that these documents needed names for reference, and maybe these names sounded better than simply calling them "1, 2, 3, and 4" but the reality is that giving them such numerical names would've been better in the sense of not deceiving the masses.
And the confusion doesn't stop there. Paul wrote some epistles, but not all of the ones credited to him are actually his works. Yes, some of the letters leading with "Paul, called to be an apostle..." were not written by Paul. It was a different world back then. No copyright laws, no printing press, no publishing houses, rampant illiteracy, etc. It was considered common practice and not in any way dishonest to attach the name of a prominent person to your work if you wanted to get it out there. And speaking of prominence, that's pretty much all Paul really had. There was no divine sign from heaven declaring him as a prophet. His greatest miracles were things like not being bitten by snakes, escaping in a basket, or surviving brain trauma. He has nothing more than the claim that he saw (or heard?) Jesus, nothing more than the claim that he was blind for a weekend. Couldn't Richard Dawkins do the same? With as much as he's invested against the church, and with his level of prominence, he could reverse course, claim divine revelation, and he would be no different than Paul. Paul was a prominent person who hated the church, then changed course, and since the church was desperate for any help they could get they were thrilled to have him. Even if we assume the anonymous gospels are true, there's no actual reason to believe that Paul was a prophet at all. All we have is a few of his works combined with other works which are polite forgeries borrowing his name, none of which were even addressed to us in the 21st century but rather to a specific audience for specific reasons which do not even apply anymore, and to top it off, it was narrowly decided hundreds of years later by a completely different culture whether these documents which had been copied and copied and copied many times over would even be included in the canon or not.
Here's the best way to look at it. As an outsider to the Jehovah's Witnesses, you might cite the fact that Charles Taze Russel was not competent in the language that he needed to know in order to produce the translations that he brought about. Yet you believe that an uneducated, illiterate Jewish fisherman by the name of Simon, whom Jesus referred to as Peter, drafted epistles in the Greek language.
Bizarre, to say the least. To be charitable to your position, we could say that Peter dictated his letters in Hebrew to someone who knew how to write Greek, but to my knowledge that's not really what the
traditions inventions hold.
Now, what does this have to do with martyrdom?
See, the martyrs are another of these Catholic
traditions inventions. You cannot show me as historical fact that any "eyewitness" apostle was martyred except Peter, but we don't actually know the line of questioning involved in Peter's martyr. You just help yourself to the assumption that it went like this:
Roman guy: Deny Christ.
Peter: No.
Roman guy: But I'll torture you if you don't.
Peter: I'm still not gonna.
(Several hours later)
Roman guy: This is it. I'm tired of whipping you. You're going on that cross if you don't deny Christ.
Peter: Crucify me upside down please.
But how do you know it didn't go like this:
Roman guy: You were preaching the gospel, weren't you?
Other Roman guy: If you deny it we'll "interrogate" you until you confess.
(Pro tip: "interrogate" means torture)
Peter: Yeah, I done did it.
The first Roman guy: So you know what happens now, right?
Peter: Yeah, I'll take the upside down version, please.
In either version of events, it might be said that Peter "died for his faith" or that his "faith was sealed in blood." This does not mean he was given the choice to deny Christ and go free. Nowhere is that claim ever even made except as a pulpit invention.
Now, maybe you do have people like Polycarp who willfully chose to die despite being given the option to deny Christ and go free. But he was not an "eyewitness" so he's not willfully dying for a lie.
The "Why die for a lie?" argument is itself a lie.