Having said that, an absence of faith *can* make some things an awful lot more difficult to accept. Questions that those of faith can answer quickly and simply will be questions that remain unanswered for those without a theological belief.
Then again, for those who cannot rationalize this type of faith, trying to have that faith can be even more damaging again.
Great question – a tough one in some ways, but NO – an absence of faith does NOT cause mental illness. The worries that can surround it can exacerbate mental illness though.