Why is FAITH so important in believing God can do things for you?

The bible says that if you have faith as a musterd seed you can move mountains. The Bible also says that if you ask with al your heart and have faith the LORD will give it to you if it improves you and not brings you down . Faith is very poweful .I Garrenty you if you pray to GOD with alll your heart and have faith he will grant you your requests that you have . Alson ofcourse he wants to grant them your his children!! but again i tell you have great faith!