Well with that in mind he'll lose more 'faith' from some parts of the people along as well. I get that it's okay to be honest (If they are) but when you're in the political scene you would want to be neutral but supportive in order to gain different types of people for you.
Well I have found that politicians need to generally be careful how they talk about religion or their personal faith, remembering how Mitt Romney being more unapologetically LDS bring criticism from at least a few groups. That, and I have found many a number of people who have, shall we say "Lost faith" in organized religion yet still have a personal belief in God. This doesn't make them atheist, and from what I've read it may simply be that Bernie may simply be one such individual possibly.