Faith plays a vital role in achieving felicity in life. Talking about faith in the field of theology is of a long history which always expressed as an important and effective subject by the intelligentsia. Their discussion is that whether faith should be rational or not. The present article studies and criticizes fideism in Islam and Christianity based on their theoretical principles (in two sections). In the first section the concept of faith in Christianity, fideism and its classifications, principles of some of the intelligentsia such as Soren Kierkegaard, criticism of fideism and rationalism will be discussed.