Isaiah 55:10-11

For as the rain and snow come down from heaven, and do not return there without watering the earth, making it bear and sprout, and providing the seed to the sower and bread to the eater, so will My word which goes out of My mouth; it will not return to Me void without accomplishing what I desire, and without succeeding in the matter for which I sent it.  

We are to speak the word of God out loud daily to activate it like a seed for our needs in our lives.  We are to be imitators of God and Jesus who spoke the word in faith, and the word spoken in faith provided a harvest or outcome.  Believing and speaking is a cornerstone of faith, that we believe in our hearts and speak with our mouths based on the word of God.