What is spiritual hunger?

Hell is full of professing Christians. 

This may sound strange, but it is true nevertheless since all the thousands are here to whom Christianity in life was but an outward thing—a habit—or even a mask, hiding an unconverted heart—all those who having heard the message of salvation listened to it complacently, but never strove to make sure of it for themselves, merely playing with God's truth, as it were, falling away in the time of temptation. 

They are hungering and thirsting now for the word once despised, but it is passed away forever. 

They know it, for some of them have been at their hopeless endeavour for years and centuries now, but they cannot resist flocking to the would be churches, listening anxiously to ministers that cannot minister.

Letters from Hell