Natural theology which is derived from general (natural) revelation, stands as a polar opposite to fideism in matters of philosophy and theology. Where natural theology asserts that people can and do gain valid knowledge of God by means of natural reason reflecting upon natural revelation, fideism asserts that God can be known only by faith. Fideism as an ism does not merely assert that faith is crucial to Christianity. The ism of fideism negates a knowledge of God via natural theology. It denies man’s ability to know God except by faith.
— R. C. Sproul, from Classical Apologetics, p. 27