^ Spock put his soul in McCoy so it would be brought to Vulcan. I don't think he had any thoughts at the time of coming back to life.
This is also what Sarek originally came to Kirk about not about bringing Spock back to life.
However Spock was a special case due to genesis he did have a chance to come back to life. That was the ritual that had not been done for a long time (dead Vulcan bodies don't fall on Genesis every day)
Of course that is t to say there wasn't a lot of mumbo jumbo in the episode. How Spock came back is more fantasy then good science fiction but oh we'll the scenes with McCoy as Spock are great so who cares.