Matthew 14:28

And Peter answered him and said, Lord, if it be thou, bid me come unto thee upon the waters.

Peter's "if" in this place is not a word of doubt but an argumentative "if" such as Christ himself used when he said, "If I go, I will come again." The true meaning is, "Since it is thou, etc." Peter, impetuous as always, dared the impossible, and with what memorable results. He actually did it, for a while, at least!