Today, we shall not look at Jesus sleeping on he boat while it sinks, nor rebuking the winds and the waves with a single word, so his disciples may be amazed (cf. Mt 8:23-27). But, today's action is no less disconcerting, whether for his first disciples or for us.
Jesus had obliged his disciples to get into the boat and go ahead of him to the other side; and after the crowds have eaten their fill, He had sent the crowd away. And, then, He went up the mountain by himself to pray. And He remained there alone. (cf. Mt 14:22-23). Without his Master, the disciples were having troubles to face the wind. It was then when Jesus came to them walking on the water.
As plain and simple people would, the disciples were terrified to see him: men do not usually walk over water, so they thought they were seeing a ghost. But they were wrong: it was not an illusion what they were looking at, but the very Lord, who was inviting them —as He did quite often— not to be afraid and trust him to awake their faith in them.
This faith was first demanded to Peter, who said: «Lord, if it is you, command me to come to you walking on the water» (Mt 14:28). With this words, Peter showed that faith consists of abiding by the word of Christ: he did not say «let me walk on the water» but he just wanted to follow what the very and only Lord could command him to do, to believe the truthfulness of the Master's words.
His doubts, however, made him reel, but they led the other disciples to bow down and confess before their Master: «Truly, you are the Son of God!» (Mt 14:33). «The group of those that already were apostles, but did not yet fully believe, when they saw the waters waving below the Lord's feet and appreciated his steps were firm through the stormy waves (...) they believed Jesus was the true Son of God, and accepted him as such» (St. Ambrose).