Mohler cites an interaction (p165 of He is Not Silent) between Spurgeon and a student at his pastor’s college:

A student . . . once asked how he could focus more clearly on bringing believers into the faith.  “Do you expect converts every time you preach?” Spurgeon asked.  The student quickly retorted, “Of course not.”  And the reply came back: “That is why you have none.”

Selah.