"Once the inner connection is grasped, all theoretical belief in the permanent necessity of existing conditions collapses before their collapse in practice -- Letter to Ludwig Kugelmann (July 11, 1868)"

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A faith without some doubts is like a human body with no antobodies in it. People who blithely go through life too busy or indifferent to ask the hard questions about why they believe as they do will find themselves defenseless against either the experience of tragedy or the probing questions of a smart skeptic. A person's faith can collapse almost overnight if she failed over the years to listen patiently to her own doubts, which should only be discarded after long reflection.

— Timothy Keller