So (ignoring of course all those xtian groups who demand total obedience to a figurehead - let's start with Catholicism and go on...) a cult is basically any religion other than Billys' version of xtianity. Plus that whole problem of dominoinists being very picky about which bits of the bible they use - persecuting gays because of a line in Leviticus, but not going after eaters of shellfish who are condemned with equal wrath a few verses on.