The Collaborative International Dictionary of English v.0.48:

Credulousness \Cred"u*lous*ness\, n.
   Readiness to believe on slight evidence; credulity.
   [1913 Webster]

         Beyond all credulity is the credulousness of atheists.
                                                  --S. Clarke.
   [1913 Webster]

WordNet (r) 3.0 (2006):

credulousness
    n 1: tendency to believe too readily and therefore to be easily
         deceived [syn: credulousness, gullibility]