Warning: contains spoilers for New Fantastic Four #1!
Marvel has finally explained pressing questions raised by One More Day is finally answered in New Fantastic Four #1, thanks to Ghost Rider and the single greatest weaknesses of Marvel's demons.
In 2007, Spider-Man was happily married to Mary Jane; their marriage had its ups and downs like any other married couple, but they were overall enjoying their lives together. That changed with Spider-Man: One More Day, written by J. Michael Straczynski and Joe Quesada, in which Aunt May is shot by a stray bullet (the shooter meant to kill Peter). After he is refused help from Iron Man and Doctor Strange, Peter is approached by Mephisto who offers a deal: Aunt May will avoid death in exchange for Peter's marriage to Mary Jane. Peter accepts and the marriage is erased, along with his memories of the entire event.
In New Fantastic Four #1, written by Peter David with art by Alan Robinson, Spider-Man accompanies Wolverine, the Hulk and Ghost Rider on a mission to Las Vegas (each member of the team thought of the city and desired to journey there, and Ghost Rider believes a supernatural element is at work because of this). On the ground, the four are immediately attacked by an angry mob of city residents, and Ghost Rider later explains the throng were all possessed by demons. "Most demons don't like doing that," explains Ghost Rider "because it brings them too close to human souls, which are anathema to them."
The demonic revelation finally sheds light on exactly why Mephisto decided against asking for Peter Parker's soul. Clearly, even being in the mere presence of human souls hurts demons of all sorts, and acquiring his marriage allows Mephisto to punish Peter while also allowing himself to be as far away from souls as possible. Mephisto has been seen acquiring souls before, so either Mephisto has grown weaker over time or the human soul has grown stronger in the Marvel Universe.
Spider-Man: One More Day may be hated by the Marvel fan community, but at least one oddity of the story makes slightly more sense. Peter's soul was never in jeopardy because even demons like Mephisto cannot stand to be close to human souls. Thus, even when Spider-Man is utterly defeated, his humanity saves him.
