John Calvin Commentary John 6:19

"When therefore they had rowed about five and twenty or thirty furlongs, they behold Jesus walking on the sea, and drawing nigh unto the boat: and they were afraid." — John 6:19 (ASV)

They were terrified. The other Evangelists explain that the cause of that fear was that they thought that it was an apparition (Matthew 14:26; Mark 6:49). Now, it is impossible not to be seized with consternation and dread when an apparition is presented before our eyes, for we conclude that it is either some deception of Satan or some bad omen that God sends us.

Besides, John here shows us, as in a mirror, what kind of knowledge of Christ we can obtain without the word, and what advantage can be gained from that knowledge. For if he presents a simple demonstration of his divinity, we immediately fall into our imaginations, and every person forms an idol for himself instead of Christ.

After we have thus wandered in our understanding, this is immediately followed by trembling and a confused terror of heart. But when he begins to speak, we then obtain clear and solid knowledge from his voice, and then also joy and delightful peace dawn upon our minds. For there is great weight in these words: