A common modus operandi for cults is to cut off the cult members from the outside by fostering an 'us and them' mentality. Social connections with non-cult members are typically discouraged, as these can threaten the dominance of the cult's world-view in the indoctrinated individual. Scientology attempts to slowly ease recruits into the cult mindset, since their core beliefs are too radical to find purchase in many individuals who've not already been heavily indoctrinated in preparation for them. By the time Scientologists are presented with the 'inner truths' of Scientology, they've spent tens of thousands of dollars on 'auditing', put in thousands of hours of work for the Church of Scientology and have been cut off from their non-Scientologist friends and family, making them highly susceptible to even completely bizarre stories about genocidal alien overlords, H-bombs, volcanoes and superpowers that'll emerge if they get rid of enough alien ghosts that are clinging to them.
The line between a cult and a major organisation like a religion or radical political movement can be quite blurred, too. Many major religions and some political parties act in much the same way as cults, discouraging their members from communicating with non-members or leaving the organisation. In Islam, it's often a death sentence to leave, communist parties have often made it illegal to not be a member in countries where they've taken power and most religions have a strong peer pressure component to control members to varying degrees. That's not to say that all cults/religions/political parties behave in this way, but it's very common for them to do so.
