User blog comment:Twilight Despair 5/Hell Trial Theories/@comment-5619706-20130330041257/@comment-97.102.247.111-20130401005618

ADAM can be the innocent soul rescued; BELA can be the soul prevented from turning into a demon or restored after the fact.

Lucifer saying that was just Sam's hallucination repeating what Sam himself believed, what was probably drilled into him by the real Lucifer and Michael: "You can't get out, you can never get out, it's impossible." But it is canonically possible to enter Lucifer's cage--Cass did it to save Sam's body, and Death did it to save Sam's soul. It's just extraordinarily difficult for most beings--and nobody seems particularly bothered about doing it anymore. And while I agree with you that we don't know if souls CAN turn into demons inside the cage (I think that otherwise Sam would have turned into a demon), it's never implied in canon that they CAN'T.