If knowledge of God cannot be attained through a reasoning process, like a proof in geometry, then how is it attained? The fideist claims that it is in fact impossible to attain rationally, but must be apprehended by a totally nonrational leap of faith. This is the claim of fideism. It is an exaggeration of pessimism.
Following is a critique of fideism:
"Fideism makes for strange theological bedfellows. Though crass forms of fundamentalism and dialectical thinkers like Barth and Bultmann are worlds apart with respect to their understanding of the content of Christianity, their method of faith is essentially the same. Fideism is no respecter of persons, finding advocates among liberals and conservatives, as well as virtually every other stripe of theology. The common denominator is the conviction that faith does not rest upon reason but functions prior to and independent[ly] of rational evidence.
"Presuppositionalists insist that all traditional apologetics, such as that of [Jonathan] Edwards, is futile, and worse than futile -- blasphemous. Traditionalists may mean well, say presuppositionalists, but they do not think well.... This serious indictment is based on several allegations. First, the traditional procedure, according to presuppositionalism, starts off on the wrong foot -- autonomy. Traditionalists think that they can begin their thinking without God, being a law to themselves. They think themselves to be the measure of all things including the God who made them.
"Second, even if this false apologetic ever could get started -- as it cannot -- it could never reach its goal. For if it begins with man, it is going to end with man, it will never end with God. The only way to end with God is to begin with God.
"Third, the presuppositionalist insists that if the traditionalist did reach his goal, it would be an unworthy goal. The evidentialist cannot begin where he begins, cannot reach his goal if he does begin there, and if he thinks that he as reached his goal, that very conviction is his ultimate folly. Manifestly, beginning with a finite start and going through finite steps, he must finish at a finite end."
R.C. Sproul, J. Gerstner, A. Lindsley, Classical Apologetics: a Rational Defense of the Christian Faith and a Critique of Presuppositional Apologetics, Academie Books, Zondervan (1984).