People "of faith" tend to point to this idea of "well, you can't be completely sure of ANYTHING" as an argument for faith being a necessary part of life. And while I agree with this to the extent that we can't, truly, be sure of the objectivity of the universe and "reality" our sense present us with, there are definitely LEVELS OF UTILITY that can be applied versus the "realities" we are either presented with (such as the universe as interpreted by our senses), or that we present ourselves with (such as religious belief or ideas of "spirituality").
I would say that the level of utility that something like "seeking water for survival" achieves is far, far greater than the level of utility of seeking religious experience or understanding. My point being - while you can't be entirely sure of the objectivity of the water your senses detect or the elements of your religious belief, "water" wins out as being more important to ongoing bodily/intellectual/emotional processes, hands down. Looking at a spectrum of these various levels of utility one could ascribe all of the things we "can't be sure about" - religion, belief in the supernatural, etc. would be far removed from the "more practical" end of the scale.