I've noticed that, among a large cohort of non-Christians and especially atheists on here, many of them take disbelief to what I would consider an extreme. There are many issues so this will likely be a wide-ranging thread.
Out of curiosity, have you ever noticed a theist take their beliefs to what one might consider an extreme? For example, that Adam and Eve were real people, and the actual first humans? Or that the entire surface of the earth was once completely flooded, and the ancestors of every living animal today were saved on a big boat? I only ask because you don't mention that.
I. 'Christian martyr stories are made up.' Why? Yes, there are many apocryphal tales but it is absolutely true that Christians were at times persecuted and put to all kinds of terrible deaths by the Roman state. These figures are exaggerated but why should this mean that the whole idea behind Christian martyrs be questioned?
Many stories, both religious and non-religious are made up. This happens for a variety of reasons -- sometimes to make a point (Jesus, after all, invented parables for that purpose). Others are exaggerated (there's much truth in Homer's Odyssey and Iliad, and there's likewise truth in Virgil's Aeneid. Troy existed, but who really believed that before Schliemann? So did Carthage, whether or not it was founded by a queen called Dido.
As
@Father Heathen pointed out, many people have died for their beliefs -- beliefs of all kinds. Many Americans died for a belief in democracy, which doesn't mean that democracy will hold out forever in the U.S. Many Christians died for Catholicism, and many died for Protestantism. How does their dying make either any more "the True Religion?"
II. 'Paul was xyz.' (A Roman spy, a false Christian, didn't really see Jesus etc.) Please prove it. Paul probably had more enemies than friends, but the same might be said of Jesus.
Paul did not meet Jesus, as the Book of Acts makes clear in the Bible. Paul only converted some years (maybe 4-7) after the crucifixion, but he certainly met some of the Disciples.
We hear two accounts of Paul's conversion on the road to Damascus: one in Acts 9, and the second Acts 22:
The men who were traveling with him stood speechless because they heard the voice but saw no one. (Acts 9:7)
Now those who were with me saw the light but did not hear the voice of the one who was speaking to me. (Acts 22:9)
But throughout the authentic Epistles (yes, Paul was real, and wrote letters that virtually all scholars agree were by him), Paul downplays every aspect of Jesus's life on earth, isn't interested in Mary and Joseph, nor Pontius Pilate, and even treats the Crucifixion itself as if it happened in some mystical realm.
III. 'Jesus didn't exist.'
I have always said that it is most likely that Jesus did exist. What I haven't agreed with, on the balance of the evidence and of my own particular reason, that he was divine, born of a virgin, nor the "Son of God." Those things do not bear real scrutiny.
IV. Sources that never seem to be good enough. Yet other histories are not questioned (ex. our best information for Alexander the great comes about 200 years after the fact and almost nothing contemporary survives). Ancient written histories and biographies are full of what modern folks would now consider nonsense and are yet still cited as acceptable histories and especially biographies, yet when one goes to the Gospels all of a sudden it's different, despite the fact that the Gospels are now squarely classed as Greco-Roman biography written in the style of every other such biography (ex. miraculous birth narratives, missing out childhoods, not in chronological order etc.)
Lots of "histories" and "sources" are questioned -- all the time, as you've been shown by others in this thread. We don't accept Homer or Virgil as historians, but as storytellers, doing what most storytellers do: taking such real historical themes as they may know, and embellishing them for their audiences. And we invariably discount the stories of mythical beings like Gorgons and Circe and the Cyclops. We discount tales of men being turned into pigs, or of Gilgamesh and Enkidu killing "the Bull of Heaven." There was no "Bull of Heaven," though there was indeed an Uruk. And it is in the Epic of Gilgamesh that we first find the story of the flood, and of Noah (named Utnapushtim in the Gilgamesh).
The minute something is classed as 'religious' it seems far too many people are willing to write it off as a complete waste of space.
I think that you will find most of the time, it is when something classed as "religious" is used to hurt other people that a lot of atheists get up in arms. For example, the present "culture war" in the United States, primarily aimed at LGBTQ+ people, is just about entirely fuelled by religion -- and although I'll bet you haven't considered this, it is right now making a lot of LGBTQ+ people very afraid. That, for some of us who can't justify it with religion, is a BAD THING. And we say so out loud.