Okay, this thread, this whole question, has had me lost. I cannot see how someone could, on a whim, give up their religion, their beliefs, for
any reason no matter how honorable. You bring up people leaving their religion to "find the truth", but that indicates that they are lacking a feeling of "truth" from the religion they are in and have lost belief in it. If one believes something, they believe it. They are convinced of it. From teachings, experiences, and so on, they are psychologically convinced of something. One cannot ever simply
choose to not hold a certain belief/religion anymore and it be honest. It would be a lie, as
@Smart_Guy already eluded to having to say. If you believe that jazz music is atrocious you cannot simply choose, on mere whim, because someone asks you to, to suddenly love jazz music. If one believes the colors in the sky at sunset are breathtakingly beautiful you cannot make them just decide they are the ugliest things they have ever seen. If you believe in something, truly believe it, there are reasons that have brought you to believing it and unless you chip away at every single reason they have for believing you cannot get them to not believe simply by telling them to do so.
Frankly, the whole premise of having to give up, as if it is a ratty doll to chuck away, one's religion/belief in order to save someone's life is something I find ludicrous. One is not being selfish by saying "no" to this absurd question, one is merely being honest. Anyone who says they could or would "give up" their religion/belief to save a life is lying to look self-sacrificing. Either that or they really don't honestly believe what they profess to in the first place.