I've just started reading Heschel's "God in Search of Man" and came across a really great discussion about rationalism and religion.  The emphasis is mine and reflect the thoughts that caused me to ponder:

It is impossible to define philosophy of religion as an attempt to support a rational basis for religion, because such a definition implicitly identifies philosophy with rationalism.  If rationalism were the sign of a philosopher, Plato, Schelling, William James, and Bergson would have to be disqualified as philosophers. Rationalism, according to Dewey, "precludes religious faith in any distinctive sense.  It allows only for a belief that is unimpeachable rational inference from what we absolutely know."

Extreme rationalism may be defined as the failure of reason to understand itself, its alogical essence, and its meta-logical objects.  We must distinguish between ignorance and the sense of mystery, between the subrational and the super-rational.  The way to truth is an act of reason; the love of truth is an act of the spirit.  Every act of reasoning has a transcendent reference to spirit.  We think thorugh reason becasue we strive for spirit.  We think through reason because we are certain of meaning.  Reason withers without spirit, without the truth about all of life.

Reason has often been identified with scientism, but science is unable to give us all the truth about all of life.  We are in need of spirit in order to know what to do with science.  Science deals with relations among things within the universe, but man is endowed with the concern of the spirit, and spirit deals with the relation between the universe and God.  Science seeks the truth about the universe; the spirit seeks the truth that is greater than the universe.  Reason's goal is the exploration of objective relations; religion's goal is the exploration and verification of ultimate personal relations.  

A challenge is not the same as a clash, and divergence does not mean a conflict.  It is part of the human condition to live in polarities.  It is an implication of our belief in one God to be certain that ultimately reason and revelation are both derived from the same source. Yet what is one in creation is not always one in our historic situation.  It is an act of redemption when it is granted to us to discover the higher unity of reason and revelation.

The widely preached equation of Judaism and rationalism is an intellectual evasion of the profound difficulties and paradoxes of Jewish faith, belief and observance.  Man's understanding of what is reasonable is subject to change.  To the Roman philosophers, it did not seem reasonable to abstain from labor, one day a week.  Nor did it seem unreasonable to certain plantation owners to import slaves from Africa into the New World.  With what stage in the development of reason should the Bible be compatible?

For all the appreciation of reason and our thankfulness for it, man's intelligence was never regarded in Jewish tradition as being self-sufficient.  "Trust in the Lord with all thy heart and do not rely on thine own understanding" (Proverbs 3:5).  "And thou has felt secure in thy wickedness, thou has said: no one seeth me; thy wisdom and thy knowledge led thee astray, and thou has said in thy heart:  I am, and there is no one else beside me"  (Isaiah 47:10).

Some of the basic presuppositions of Judaism cannot be completely justified in terms of human reason.  Its conception of the nature of man as having been created in the likeness of God, its conception of God and history, of the election of Israel, of prayer and even of morality, defy some of the realizations at which we have honestly arrived at the end of our analysis and scrutiny.  The demands of piety are a mystery before which man is reduced to reverence and silence.  Reverence, love, prayer, faith, go beyond the acts of shallow reasoning.

We must therefore not judge religion exclusively from the viewpoint of reason.  Religion is not within but beyond the limits of mere reason.  Its task is not to compete with reason but to aid us where reason gives only partial aid.  Its meaning must be understood in terms compatible with the sense of the ineffable.

The sense of the ineffable is an intellectual endeavor out of the depth of reason; it is a source of cognitive insight.  There is, therefore, no rivalry between religion and reason as long as we are aware of their respective tasks and areas.  The employment of reason is indispensable to the understanding and worship of God, and religion withers without it.  The insights of faith are general, vague and to the mind, integrated and brought to consistency.  Without reason faith becomes blind.  Without reason we would not know how to apply the insights of faith to the concrete issues of living.  The worship of reason is arrogance and betrays a lack of intelligence.  The rejection of reason is cowardice and betrays a lack of faith.

Herschel, A J (1955).  God in Search of Man.  Farrar, Straus and Giroux.  Pages 18-20.