For they had not understood what had happened with the loaves, because their heart was hardened.
The Evangelist gives a profound explanation for their amazement. They still had not grasped the true meaning of the miracle of the loaves. If they had understood that Christ is the Lord who brings abundance for thousands from five loaves, they would have trusted more easily that He also has authority over the wind and the sea. Their heart was hardened not in the sense that they were unbelieving enemies, but that they were still heavy and slow in spiritual understanding. This is a greatly humbling truth even for believers. A person can see God’s great works and still be slow to understand their significance. For this reason God often repeats His testimonies until the heart is opened for a fuller knowledge.