According to the Catholic Catechism, two kinds of doubt have to be distinguished. The first is involuntary doubt. We see this in Mary in the Gospels when she said "How can this be, since I do not know man?"" It is when one has trouble believing what we are asked to believe, we need help and explanation. "Help my unbelief."
Voluntary doubt is a willful decision to not believe something that has been revealed by Revelation.
You seem to confuse doubt, which can be of either kind, with willful unbelief. For example, the current atheists (Christopher Hitchens, Richard Dawkins, etc.) do not doubt: they are sure there is no God. How are they sure? I don't know. But their doubt is of the second kind, not the first kind.
Doubt of the first kind is human, part of the human condition. Doubt of the second kind is often sinful, obstinate, dogmatic in its unbelief. It's often said that Mother Teresa doubted, but only in the first sense, not the second sense. And the difference is critical.
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