24Meanwhile, the disciples were in trouble far away from land, for a strong wind had risen, and they were fighting heavy waves. 25 About three o’clock in the morning Jesus came toward them, walking on the water. 26 When the disciples saw him walking on the water, they were terrified. In their fear, they cried out, “It’s a ghost!”
27 But Jesus spoke to them at once. “Don’t be afraid,” he said. “Take courage. I am here!”
28Then Peter called to him, “Lord, if it’s really you, tell me to come to you, walking on the water.”
29 “Yes, come,” Jesus said. So Peter went over the side of the boat and walked on the water toward Jesus.
30 But when he saw the strong wind and the waves, he was terrified and began to sink. “Save me, Lord!” he shouted.
31 Jesus immediately reached out and grabbed him. “You have so little faith,” Jesus said. “Why did you doubt me?” (Matthew 14:24-31)

Fishing boats in a harbourHoly Bible, New Living Translation

Bearing in mind that Peter had actually had the faith to step out of the boat, it seems a harsh judgement to say that he had so little faith. Obviously, Jesus is measuring Peter’s faith in a linear way – he is saying that he didn’t have enough faith to last.

Peter stepped out of the boat, walked on water, and ran out of faith!

Don’t run out – it will only get you down.