Source : Webster's Revised Unabridged Dictionary (1913)

Credulousness \Cred"u*lous*ness\, n.
   Readiness to believe on slight evidence; credulity.

         Beyond all credulity is the credulousness of atheists.
                                                  --S. Clarke.

Source : WordNet®

credulousness
     n : tendency to believe too readily and therefore to be easily
         deceived [syn: {gullibility}]