Albert Barnes Commentary Matthew 14:28-31

"And Peter answered him and said, Lord, if it be thou, bid me come unto the upon the waters. And he said, Come. And Peter went down from the boat, and walked upon the waters to come to Jesus. But when he saw the wind, he was afraid; and beginning to sink, he cried out, saying, Lord, save me. And immediately Jesus stretched forth his hand, and took hold of him, and saith unto him, O thou of little faith, wherefore didst thou doubt?" — Matthew 14:28-31 (ASV)

And Peter answered, and so on. Here is an instance of the characteristic ardour and rashness of Peter. He had less real faith than he supposed; more ardour than his faith would justify: he was rash, headlong, incautious, really attached to Jesus, but still easily daunted, and prone to fall. He was afraid, therefore, when in danger, and sinking, cried again for help. Thus he was allowed to learn his own character, and his dependence on Jesus: a lesson which all Christians are permitted to learn by dear-bought experience.