A deathbed conversion is a sudden change in a person's beliefs late in their life, possibly during a terminal illness. This change is usually to Christianity from some other belief system or atheism.
Apologists occasionally use deathbed conversion stories of many atheists and humanists to illustrate the importance of accepting Christianity before death. Many, if not all, of these conversion stories are myths.
A deathbed conversion story is essentially an "I used to be an atheist" argument, but told second-hand. As there, the important thing is not the timing of the conversion, but the reasons behind it.
If no reason for the conversion is given, then the argument essentially becomes: "This person was like you. This person found sufficient reason to convert. Therefore, whatever those reasons were, they should be enough to convince you, and you should convert."
Deathbed conversion stories, by virtue of their timing, usually involve either the threat of hell or the promise of heaven, either explicitly or implicitly. However, neither wanting heaven to exist nor the fear of hell makes them exist.
A person who is about to die is likely not in a rational frame of mind, and therefore the reasons behind a deathbed conversion deserve additional scrutiny. We would not accept an argument like "John was drunk and despondent over the death of his wife when he got a tattoo, therefore you should get a tattoo as well", and neither should we accept "John was terrified that God would condemn him to hell, so he converted, and so should you."
- Charles Darwin - Many tracts and sermons have included a story of Charles Darwin's deathbed conversion, but there is no evidence to support this. The "Lady Hope" story, in which a well-meaning woman is asked to sit in with the ailing Darwin during his last days, was published in a Baptist newspaper in 1915. Darwin's family denied the story.
- Carl Sagan - "Contrary to the fantasies of the fundamentalists, there was no deathbed conversion, no last minute refuge taken in a comforting vision of a heaven or an afterlife. For Carl, what mattered most was what was true, not merely what would make us feel better. Even at this moment when anyone would be forgiven for turning away from the reality of our situation, Carl was unflinching. As we looked deeply into each other's eyes, it was with a shared conviction that our wondrous life together was ending forever." - Ann Druyan, Epilogue to Billions and Billions: Thoughts on Life and Death at the Brink of the Millennium