Luke 6:25

A good man out of the good treasure of his heart bringeth forth that which is good; and an evil man out of the evil treasure of his heart bringeth forth that which is evil: for of the abundance of the heart his mouth speaketh.

God knows the heart of man, whether it be good or bad.  Man looks on the outside and says, we can do nothing with this person.  God looks on the inside and says I can do great things with the person.  The heart is key to being used by God.  A faithful, humble heart working with God will move a mountain.