People who remain in cults, as I did, experience doubts just like non cult-members do. The difference is that cult members put up walls against the unbearable logic of these doubts. Against all reason they persist in their commitment. They have so totally invested in the system—which they believe meets their three cosmic needs of assurance of salvation (security), love, and significance—that they will ignore such doubts or explain them away in order to remain consistent with their previous commitment.
As Cialdini alludes, it is not always the act of hard-thinking that discourages cult members from thinking critically about their group, but rather the consequences of such thinking. To admit that one’s group is a cult carries serious consequences: it means that you have been deceived, that you have judged others wrongly, that you have treated disagreeing family members disgracefully, and that you have misunderstood the character of God.
A former UBF member also explained this as the "sunk costs" problem.