1) Credulity 2) Gullibility 3) Naivete 1) Gullibility 2) Naiveness 3) Naivete 4) Naivety 
• (n.) Readiness to believe on slight evidence; credulity.
Cred'u·lous·ness noun Readiness to believe on slight evidence; credulity. « Beyond all credulity is the credulousness of atheists. S. Clarke. » 
[n] - tendency to believe too readily and therefore to be easily deceived
credulous, credulousness 1. Ready or too easily disposed to believe. 2. Overly ready to believe; apt to believe on weak or insufficient grounds. 3. Someone is credulous when he or she is all too ready to believe something or someone.
gullibility noun tendency to believe too readily and therefore to be easily deceived