I think it's important to separate the shepherds from the flock. Is it possible to gain salvation from a tainted source? I expect it is, because God is a loving God. And besides, it is impossible for me to judge what lives in the heart of another. So I accept the fact that my Mormon and Islamic fellow citizens are children of God, even though I believe that their prophets are false. I suppose those are fighting words to some.
But it goes deeper for me. Suicide is a sin. Murder is a sin. When the followers of Jim Jones drank the potion in the jungle, did they lose salvation? I cannot judge. I do not know what was in their hearts. Their leader was, beyond doubt, a false prophet. The same was true of David Koresh. The fruit of their prophecy and revelation was not life, but death. Through their guile and deceit they obscured the truth, but that does not mean that all their followers shared their taint.
The same is true for Islam and the the Latter Day Saints.