The disciples have just seen Jesus doing something that only God can do. They know the Old Testament Scripture which testifies to the fact that the Lord God is the one who made and controls the sea (see Genesis 8:1; Job 26:12; Psalm 65:7; Psalm 74:13–14; Psalm 89:9; Psalm 93:3–4; Psalm 104:5–9; Psalm 106:9; Psalm 114:3; Isaiah 50:2; Nahum 1:4). On other occasions, they would have sung from Psalm 107:23–32 about how sailors on the waters would pray to the Lord in the midst of a storm and he would calm the seas. The disciples are filled with great fear because they are beginning to realize that the person who is with them in the boat has the power and authority of God.
41 And they were filled with great fear and said to one another, “Who then is this, that even the wind and the sea obey him?”