7 This faith if we keep we shall be free from condemnation, and shall be adorned with all kinds of virtues. For so great is the strength of faith, as even to buoy men up in walking on the sea. Peter was a man like ourselves, made up of flesh and blood, and living upon like food. But when Jesus said, Come, he believed, and walked upon the waters, and found his faith safer upon the waters than any ground; and his heavy body was upheld by the buoyancy of his faith.
But though he had safe footing over the water as long as he believed, yet when he doubted, at once he began to sink: for as his faith gradually relaxed, his body also was drawn down with it. And when He saw his distress, Jesus who remedies the distresses of our souls, said, O thou of little faith, wherefore did you doubt? And being nerved again by Him who grasped his right hand, he had no sooner recovered his faith, than, led by the hand of the Master, he resumed the same walking upon the waters: for this the Gospel indirectly mentioned, saying, when they had gone up into the ship. For it says not that Peter swam across and went up, but gives us to understand that, after returning the same distance that he went to meet Jesus, he went up again into the ship.
Source: Catechetical Lectures (New Advent)