"But when once Christ had called him, Peter had no alternative: he must leave the ship and come to Him. In the end, the first step of obedience proves to be an act of faith in the word of Christ.
But we should completely misunderstand the nature of grace if we were to suppose that here was no need to take the first-step, because faith was already there.
Against that, we must boldly assert that the step of obedience must be taken before faith can be possible. Unless he obeys, a man cannot believe."
-- Dietrich Bonhoeffer