John Chrysostom

So did Christ at that time also, and did not discover Himself before they cried out. For the more intense their alarm, the more did they welcome His coming. Afterward when they had exclaimed, it is said, "Straightway Jesus spake unto them, saying, Be of good cheer, it is I; be not afraid." This word removed their fear, and caused them to take confidence. For as they knew Him not by sight, because of His marvellous kind of motion, and because of the time, He makes Himself manifest by His voice.

Homily on the Gospel of Matthew 50 ↗

Theophylact of Ohrid

He first strengthens their resolve by saying: It is I Who can do all things; take courage.

Commentary on Matthew ↗

Jerome

Immediately Jesus spoke to them, saying: Have confidence, I am, do not be afraid. He takes care of what was first in the matter and commands those who fear, saying: Have confidence, do not be afraid. And what follows, I am, does not add who he is, either from a voice they could recognize, who spoke through the darkness of the obscure night, or they repeated that he himself is the one who spoke to Moses: Thus you shall say to the children of Israel: He who is, has sent me to you (Exod. III, 14).

Commentary on Matthew ↗

Thomas Aquinas

And immediately he spoke to them etc. Here the assistance is presented. Because they were in darkness, he gives certainty; and he does three things. First, he reassures with words; second, Peter seeks a sign by deeds; third, it is granted to him. He had presented three things: the agitation of fear, the falsity of opinion, and despair; and against these three he acts, because immediately he spoke to them. Hence when someone cries out to the Lord, if there is need, he comes immediately; Isa. …

Commentary on Matthew ↗