Take courage, it is I, do not be afraid. The above words by Jesus must have come as a relief to his disciples who were almost dying of fright at the sight of Jesus (whom they assumed was a ghost) walking on the sea. Even though most, if not all, of them were used to dealing with turbulence and all the other emergencies at sea, the sight of a ghostly figure walking on water at that particular time of the night was too much for them to bear. And when they cried out (not to anyone in particular - it was just a natural reaction), Jesus heard their cry and came to their rescue. ‘ It is I, do not be afraid,’ said Jesus to the frightened disciples. He entered the boat and the sea calmed down.
Jesus felt the terror that the disciples were undergoing. He didn’t wait for them to open their eyes in order to recognize him (fear had overwhelmed them). He reached out to them through his word. And while Mark would fault the disciples for how they handled the situation, Jesus’ words were not condemnatory. His first move was to assure them that all was well, that they were not facing death. And even though it was not until Jesus stepped into the boat that the wind calmed down, I believe the disciples had already recovered from their fright at the calming words of Jesus. 
The mystery that we are celebrating during this season of Christmas is the mystery of the word of God coming into our lives to calm our fears and to reassure us that all is well. Jesus comes into our lives to tell us not to be afraid for he has come to drive away the darkness that causes our fear. He tells us not to be afraid because he has come to give life to the world.