John Chrysostom

"It is I, be not afraid." As He spake the word, He cast out fear from their souls. But at another time not so; wherefore Peter said "Lord, if it be Thou, bid me to come unto Thee." (Matt. xiv. 28.) Whence then was it that at that time they did not straightway admit this, but now were persuaded? It was because then the storm continued to toss the bark, but now at His voice the calm had come.

Homily on the Gospel of John 43 ↗

Theophylact of Ohrid

When they were in danger, He appears before them and by His word drives away their fear, and produces calm upon the sea, and thus creates a twofold calm—He quiets the disturbance in their souls and the tumult on the sea. And now many of us find ourselves in mental darkness and in danger of drowning in the mental sea. But let us receive Christ to ourselves, and we shall be completely delivered from misfortunes. …

Commentary on John ↗

Bede

He does not say, I am Jesus, but only I am. He trusts to their easily recognising a voice, which was so familiar to them, or, as is more probable, He shows that He was the same who said to Moses, I am that I am (Exod. 3:14)

Catena Aurea by Aquinas ↗

Thomas Aquinas

Secondly, we see Christ encouraging them against two dangers. First, they are encouraged against the danger to the faith in their intellect when he says, It is I, to eliminate their doubts: "Look at my hands and my feet! It is really me" (Lk 24:39). Secondly, Christ encourages them against the danger of fear in their emotions, saying, Do not be afraid: "Do not be afraid when they are present" (Jer 1:8); "The Lord is my light and my salvation, whom shall I fear" (Ps 26:1).

Commentary on John ↗

Bonaventure

"But he said to them: It is I," namely, who protect you: "do not be afraid." When the Lord is present, there is nothing to fear, according to that verse of the Psalm: "The Lord is my protector: of whom shall I be afraid?" And from this they took confidence.

Commentary on John, Chapter 6 ↗