The problem with the sincerity argument is, as Jacob pointed out, that many people do evil while thinking it's right. I.e. rapists, murderers, Nazis, pedos. I do think that God is not restricted to one religion, however, and that it is possible for people of nearly all religions to go to heaven if they follow the most powerful commandment of Christ "Love one another as I have loved you". Cult members, IMO, are not really in their right minds and so it's difficult to know what will happen to them. Probably a spot of purgatory
