Crisis of faith is a term commonly applied to periods of intense doubt and internal conflict about one's preconceived beliefs or life decisions. A crisis of faith can be contrasted to simply a period of doubt in that a crisis of faith demands reconciliation or reevaluation before one can continue believing in whichever tenet is in doubt or continuing in whatever life path is in question - i.e., the crisis necessitates a non-compromisable decision: either sufficiently reconcile the cause of doubt with the belief or decision in question, or drop the belief.

Religious doubt could lead to anxiety over the doubter’s supposed eternal future. People experiencing religious doubt or total deconversion can suffer if they fear Hell might exist. This can apply to atheists or agnostics. The friends or relatives of freethinkers can also experience distress over the supposed eternal future of a loved one. Certain religious beliefs can lead to unhappiness. Many religious adherents derive happiness from their religion. Similarly many freethinkers derive happiness from being able to decide philosophical and moral issues for themselves.

The concept of a crisis of faith is most commonly applied to religious beliefs, since faith is a fundamental tenet of many religions and the gravity of religious decisions is perceived by many to be great enough to aptly describe a period of extreme doubt as a "crisis". It can also be applied to doubts over influential life decisions, such as doubts over a chosen career, a marriage partner, etc.