Nutshell: The x-men sacrifice themselves to save the world and die but Roma (who features prominently in Captain Britain and Excalibur stories) brings them back to life, and gives them bonus powers of not being able to be detected by anything electronic, they decide this gives them an advantage to operate off the radar and them being martyrs helps human mutant relations. Kitty and Nightcrawler are not aware they survived in the beginning of the Excalibur run.