The common reason why people think unbelief is problematic is because it stands in opposition to belief, that unbelief is a stance that prevents us from believing. However the problem with unbelief lies precisely in the opposing position: namely, unbelief is required in order to support and sustain belief. In short it actually enables us to continue in our belief. For this reason the destruction of unbelief involves the short-circuiting of belief. A short-circuiting that helps us move beyond religious belief into faith.
To flesh this out we can look at how belief operates in classically fundamentalist communities. For here we find that the explicit beliefs of the group continue to exist precisely because they are supported by a disavowed unbelief. For instance congregations might claim that if you show no doubt God will come through, or that we are much better off in the next life, or that some of our loved ones are going to an eternity of punishment. Yet the very way these views are sustained is through the disavowed unbelief that supports them. In such communities people still call the ambulance if their child is having a severe seizure, they don't shoot people to hasten their journey to heaven (or show pleasure if held at gunpoint), or act as one would who knew that most of the people around them are on the verge of unending torture. Of course there are exceptions to this and, as we will mention below, it is their naïve belief that is more of a threat to the fundamentalist community than unbelief.
This gap is sometimes acknowledged within fundamentalist communities and bemoaned, for the fantasy in operation is that what is needed is precisely more belief; that the community needs to be challenged to take their beliefs more seriously. The issue here however is that this fantasy can't be enacted without profound problems. So, in order to protect themselves from what would happen should they actually believe more they need to construct barriers: the devil is at work, we need to pray more, we need to attend a conference to make us better warriors for Christ etc.
This structure is what we see at work if we imagine someone saying to a friend, "grab me when I go to defend my girlfriend's honor by attacking that jerk at the bar." What happens here is that the person has to construct their own external barrier (the friends intervention) to protect themselves from an encounter with the true barrier: that they are not able to defend her honor. That they will end up getting beaten to a pulp.
This is why fundamentalist communities are not threatened by the anemic liberal claim that they believe too much. This is what they want. This is the fantasy that sustains them. Rather the truly radical claim is not that they believe too much but rather that they don't believe enough. That their belief is sustained by a disavowed unbelief. What one can do is challenge them to believe more.
For the unbelief allows the communities to gain the psychological pleasure from the beliefs that they hold (treating them as a security blanket) without having to confront the horror of them.
This is why the people who leave fundamentalist communities are often not the ones who don't take it seriously enough, but those who do (and who are thus confronted with the true horror of the communities beliefs). In my own experience I, along with a few friends, began to break free of religious belief precisely because we were naïve ones who took the teaching of the church more seriously than those in the church. The people who continued in a mode of disbelief were the ones that stayed because they were able to protect themselves from the trauma of actually believing their beliefs.
One poignant example I remember is that after a talk on healing in the church I briefly attended a person on my pew fell and broke his arm. I took him into a separate room and started to pray. Someone else called the leader into the room (who was also a doctor) to pray with us. He took one look at the arm and shouted that we had to call an ambulance. The point was that we had no need to do that as we fully believed the teaching he had given, the person who didn't believe (or rather whose belief was sustained by unbelief) was the leader himself.
While the supportive nature of unbelief might be obvious in fundamentalist communities the challenge is in seeing it also in operation in liberal and progressive communities. Someone might, for instance, believe that the universe is in ultimate harmony and that all things work to the good. This seems like a beautiful belief, but it is only beautiful because it is sustained by unbelief. If we removed the unbelief and fully affirmed it we would realise that it isn't too far from the views of people like Pat Robertson. We too would celebrate genocides, hurricanes etc. It is only while the belief is disbelieved that we can gain psychological pleasure from it without having to confront its horror. In short, we avoid the true nature of the belief as that which prevents us from fully embracing our human, all too human, situation.