The quality or state of being incredulous: Disbelief. (Merriam – Webster)
The quality or state of being incredulous; inability or unwillingness to believe. (Dictionary.com)
The state of being unwilling or unable to believe something. (Oxford Dictionaries)
If someone reacts with incredulity to something, they are unable to believe it because it is very surprising or shocking. (Collins Dictionary)
The feeling of not wanting or not being able to believe something: (Cambridge Dictionary)
The incredulity of Saint Thomas.
Practical causes of the incredulity of the principal philosophers.