Humans are social apes, and social pressure is quite powerful if applied correctly.
Cult indoctrination is usually a matter of finding someone who's in a sufficient level of mental/emotional turmoil for one reason or another. Typically it seems that these take the form of recent loss, fish out of water (immigrants, new college students or those recently leaving home, etc), or boredom/lack of friends. Once you find someone who's susceptible, you get two (so they're out numbered) people to offer them all the answers, and show them a prepackaged group of new family/friends. Once they start making sacrifices to get approval of the group, it's all over.
Since the chances are low that someone out door-knocking is going to catch someone in a sufficiently weakened state to be susceptible to cult indoctrination, most times this only happens when someone is exposed to a cultist on a regular basis (some friend or family member). Once life happens and they reach a susceptible state they can fall victim to the cult if they've not 'inoculated' themselves by doing research on the cult before.