Believer asks: Would the Apostles have died for what they knew was a lie?
Skeptic says: Not likely. Not for what they knew was a lie. But they may have died for what they believed was the truth.
By the way, the New Testament reports the demise of just a couple-three Apostles. In those days Jewish or Roman authorities might execute anyone who had joined an illegal cult. So some Christians were victims. Other Christians joined them in the following centuries, too. For example, Joseph Smith, the founder of the MormonChurch, claimed that an angel had told him the original way of Christianity. In 1844 an Illinois mob killed him because he practiced a new theology. He had believed or imagined it was true.
For another instance, in 1553 the Christian theologian John Calvin and local authorities executed the heretical Michael Servetus in Geneva. Servetus made the mistake of stopping in the city, so they apprehended him. He was martyred for his convictions, for what he believed or imagined was a true theology.
Further, the Greek philosopher Socrates drew the death penalty in 399 BC when a court ruled that he had badly influenced a band of youth; the band had hatched a plot to overthrow the government. Socrates' friends wanted to help him break jail. But he told them nom saying his daemon guide (which we might call a guardian angel) hadn't told him to escape. He believed that the daemon's silence was the sign that he had reached the end of his life. The supernatural guide had always been his adviser and urged him to do the right thing. He accepted his execution with his convictions intact.
Thus the Apostles weren't the only ones who had ever died in the name of a sacred belief. I say it makes little sense to get attached to a religion such as Christianity just because others have died for it.
Recap: The New Testament Apostles, Joseph Smith, Servetus, and Socrates believed in a supernatural ideal. It isn't likely they died for what they had known was a lie. But I say it’s unlikely that all of their religious beliefs were true. In general it's a matter of opinion which belief was true or false.