Faith is born through the heart but expressed through the mouth. Belief can be likened to potential energy while speaking is kinetic energy or released energy. When you speak you released what have been stored in the heart for physical manifestation. 

The effect of faith is made known through its declaration. Words that are spoken have no boundary in the Spirit realm. Your closed mouth is a proof that you don’t believe it enough. Speaking what you believe means announcing it and announcing it means calling it forth.