From Phillip Yancy's Prayer:

I write about prayer as a pilgrim, not an expert.

From the author description for Randy Alcorn's Heaven:

Dr. Randy Alcorn is a leading authority on Heaven.

The difference between a pilgrim and an expert (or authority), is that, though both spend a great deal of time and energy seeking knowledge, the expert stops at some point and says, "I have conquered this subject. I know all this is to know."

The pilgrim is ever restless. But in that restlessness, she continues to uncover truth — long after the authority has declared the case closed.