We are always exercising faith. Whether we recognize it or not, we have faith in one of two directions - it's either faith in God's Word, or faith in our own judgment, interpretation, or experience. Too often, we 'hold faith' in the wrong direction - believing not in God's Word being true, but that our experience and interpretation of our life and the world will be true. When Jesus talks about fasting, one of the layers of His meaning is 'fasting' from our own beliefs when they're opposite of what God has spoken.