When evening came” indicates in the symbolic language of the evangelist the conclusion of Jesus’ day. It is the end of his life. It is the moment in which he climbs the mountain alone, moves away from the crowds and definitely enters the world of God.
 
This is why the disciples find themselves in the dark. Darkness is the image of disorientation; doubt that captures even the most convinced believer. At times, even one who is driven by a strong faith feels alone.
 
The waters are the Old Testament images of the forces that lead to death. The Israelites were always afraid of the waters. They said that only the Lord is not afraid of the winds and the storms. He alone can hush the billows to silent waves (Psalm 107:25-30) and he is the only one who “treads on the waves of the sea” (Job 9:8).
 
If we keep this symbolism in mind, we can understand the fear of the disciples—fear of being over-whelmed by the forces of evil and death. 
 
The boat was tossed about by the waves, but the violent waves are the necessary test the disciples have to undergo if they want to come out of it with maturity.
 
Towards the end of the night Jesus appears, walking on the waves of the sea, as only God is capable of doing. 
 
The disciples do not recognise him. Their reaction is strange. What happened? Why don’t they recognise him?
 
In biblical language Matthew is describing the situation of the Christian communities of his time. They were tormented by many trials, distressed by doubts and above all disoriented by not having the master, who would have infused safety and courage in them, visibly with them.
 
The evangelist wants to enlighten them. Jesus is always close to his disciples, even the end of the world, as he has promised (Matthew 28:20), but not physically.
 
It is not easy to be aware of his presence. He becomes recognisable only with the eyes of faith.
 
The second part of the passage contains a dialogue between Jesus and Peter. It starts with a request from the apostle, “Lord, if it is you, command me to come to you on the water.”
 
The symbolic meaning of Peter’s question is that Peter, the first of the disciples, beholds the master—the Risen One—who crossed the waters of death, now walking on the sea, is in God’s world.
 
Peter knows he is called to follow him in the gift of life, but death scares him. He fears failure and asks the Lord to give him strength.
 
As long as he keeps his eyes fixed on the master, he is able to go to him. When his faith diminishes, when he begins to doubt the choice he made, he sinks and is afraid of being overwhelmed, of losing his life. It is true also for every Christian.
 
 
Father Fernando Armellini SCJ
Claretian Publications
http://bibleclaret.org/celebrating-the-word-of-god/95-blog/sunday-reflection/920-19th-sunday-in-ordinary-time-year-a-august-13-2017