This means if you are in one of these groups, you are banking an awful lot on whether the founders and leaders of your group were and are correct in their teachings; especially considering that the main bulk of Christianity disagrees with your group! That's kind of scary.
Most "cults" say they are the only ones with the "right" answers, or the whole revelation of God's word or will. They will claim that the rest of Christianity is astray because the entire church somehow became corrupt after the early apostles and their group is now the only ones restoring the truth. Usually they claim if you are not a part of their hierarchy of Church government, following their precise teachings, then you are not a part of the "real" church, you are not walking in the truth, nor are you headed for an eternity in God's favor (however they define that eternity to be spent).