credulousness

herkkäuskoisuus The characteristic or state of being credulous; credulity.
1911, Ernest Jones, "The Psychopathology of Everyday Life", The American Journal of Psychology, vol. 22 no 4 (Oct), pp. 525-6,

Credulousness is always more harshly judged than incredulousness, though they are both merely different aspects of the same fundamental failing, namely, lack of true scepticism.