Only the obedient believe. If we are to believe, we must obey a concrete command. Without this preliminary step of obedience, our faith will only be pious humbug, and lead us to the grace which is not costly. Everything depends on the first step. It has a unique quality of its own. The first step of obedience makes Peter leave his nets, and later get out of the ship; it calls upon the young man to leave his riches. Only this new existence, created through obedience, can make faith possible.

Source: The Cost of Discipleship