dubiety, dubiousness, doubtfulness, incertitude, uncertainty, distrust, suspect, question, think, wonder, more...

certainty, confidence, trust, belief, assurance

reasonable, unreasonable, irrational, logical, valid, more...

little, reasonable, much, slightest, considerable, serious, possible, self, grave, further, longer

lack of faith,   don't believe it,   put into question,   lack of certainty,   losing faith,   be skeptical of,   uncertain feeling,   to be skeptical,   slight suspicion,   to lose faith