lack is an interesting word. Lack and want are fundamentally the
   same.

   Lack of belief = want of belief (older meaning) which feeds into
   newer meaning of "want of belief".

   In short, if you lack belief, you want belief. It may have
   little to do with a 'felt urge' - which is has infiltrated the
   modern meaning of 'want'.

   Still, the root of "lack" is there. I love how words change over
   time yet retain much as well.