8 So Peter also said, “Bid me come unto You on the water.” I who dare this am but a man, but it is no man whom I beseech. Let the God-man bid, that man may be able to do what man cannot do. “Come,” said He. And He went down, and began to walk on the water; and Peter was able, because the Rock had bidden him. Lo, what Peter was in the Lord; what was he in himself? “When he saw the wind boisterous, he was afraid; and beginning to sink, he cried out, Lord, I perish, save me.”
When he looked for strength from the Lord, he had strength from the Lord; as a man he tottered, but he returned to the Lord. “If I said, my foot has slipped” (they are the words of a Psalm, the notes of a holy song; and if we acknowledge them they are our words too; yea, if we will, they are ours also). “If I said my foot has slipped.” How slipped, except because it was my own. And what follows? “Your mercy, Lord, helped me.” Not my own strength, but Your mercy. For will God forsake him as he totters, whom He heard when calling upon Him?
Where then is that, “Who has called upon God, and has been forsaken by Him?” where again is that, “Whosoever shall call on the Name of the Lord, shall be delivered.” Immediately reaching forth the help of His right hand, He lifted him up as he was sinking, and rebuked his distrust; “O you of little faith, wherefore did you doubt?” Once you trusted in Me, have you now doubted of Me?
Source: Sermons on Selected Lessons of the New Testament (New Advent)