36 This serpent then, our ancient enemy, glowing with rage, cunning in his wiles, is in the mighty sea. “Here is that Leviathan, whom You have formed to make sport of him.” Do thou now make sport of the serpent: for for this end was this serpent made. He falling by his own sin from the sublime realms of the heavens, and made devil instead of angel, received a certain region of his own in this mighty and spacious sea. What you think his kingdom, is his prison. For many say: wherefore has the devil received so great power, that he may rule in this world, and prevails so much, can do so much? How much prevails he? How much can he do? Unless by permission, he can do nothing. Do thou so act, that he may not be allowed to attack you; or if he be allowed to tempt you, he may depart vanquished, and may not gain you. For he has been allowed to tempt some holy men, servants of God: they overcame him, because they departed not from the way, they whose heel he watched, fell not....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)