Crisis of faith is a term commonly applied, especially in Western culture, 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.
Believers know that faith is bound to have its contrary temptations in the form of religious doubts. These can spring especially from the problem of evil (what Pope Benedict XVI has termed "the drama of God's darkness"). Thomas Aquinas takes up the problem of evil in the First Part of his Summa Theologica. To the objection that if an all-powerful God exists, there would be no evil in the world, his answer is that precisely because God is all-powerful, he is powerful enough to turn evil into good - even if we do not see how exactly he does this.
Since faith, by definition, in some way leads the believer beyond simple human understanding, it does not follow that intellectual difficulties automatically become difficulties in faith. John Henry Newman illustrates this point as follows: "I am far of course from denying that every article of the Christian Creed, whether as held by Catholics or by Protestants, is beset with intellectual difficulties; and it is a simple fact that, for myself, I cannot answer those difficulties. Many persons are very sensitive of the difficulties of Religion; I am as sensitive of them as any one; but I have never been able to see a connexion between apprehending those difficulties, however keenly, and multiplying them to any extent, and on the other hand doubting the doctrines to which they are attached. Ten thousand difficulties do not make one doubt, as I understand the subject; difficulty and doubt are incommensurate".
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. The friends or relatives of those dealing with a crisis of faith can also experience distress over the supposed eternal future of a loved one.
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.