Answer:
   
   What does it mean that one doubts G-d's existence? It rarely means
   that one does not believe that G-d exists, rather that a person does
   not understand what G-d does. In other words, that the way G-d runs
   the world is not comprehensible (e.g. not understanding why G-d allow
   things like famine or the Holocaust to occur). G-d does not fit into
   our limited intellect. And defining G-d to be something that would fit
   into a human ideal of what G-d should be, would mean that we are
   denying what G-d actually is: something beyond our intellect.
   
   So the answer is: we all have questions about G-d, but it does not
   prevent us from being good Jews. Because being good is not an
   end-goal, rather a process. We struggle to get better despite any
   doubts.
   
   What a Jew does is more important than what he or she believes, even
   though Maimonides included belief in G-d as one of Judaism's key
   principles. Full and complete faith (emunah sh'laimah) in particular
   is a most difficult state to achieve, but the seeds of faith find
   fertile ground in the person of one who earnestly strives to live a
   Jewish life based on the Torah's prescriptions.
   
   Note that there is an additional question of the extent to which an
   individual who doubts G-d's existance can participate in the
   congregation. This is more a problem for the individual than the
   congregation, for the congregation does not publically question one's
   belief. The individual, however, must reconcile publically performing
   actions or making professions with their internal doubts.