Isaiah 27:1

Let us earnestly endeavor, therefore, to flee every crooked and tortuous act, and let us keep our mind and the judgment of our soul as straight as a rule, in order that the praise of the Lord may be permitted to us since we are upright. In the same way the serpent, which is the author of sin, is called crooked, and the sword of God is drawn against the dragon, the crooked serpent, which makes many twists and turns in its progress.… Therefore one who follows the serpent shows that his life is crooked, uneven and filled with contrarieties; but one who follows after the Lord makes his paths straight and his footprints right.

For who is described by the designation of the "serpent" but our old enemy, at once slippery and crooked, who for the deceiving of humankind spoke with the mouth of a serpent? Of whom it is said by the prophet, "Leviathan the serpent, the crooked one"; who was for this reason allowed to speak with the mouth of a serpent, that by Leviathan's vessel humanity might learn what he was that dwelt within. For a serpent is not only crooked but slippery as well; and so because he stood not in the uprightness of truth, he entered into a crooked animal.… He spoke to man by means of a slippery animal because if one does not resist him, he secretly slips into the interior of the heart. Now "the dens" of this serpent were the hearts of wicked people.

For "Leviathan" is interpreted to be "their addition." Whose "addition," then, but the "addition" of people? And it is properly styled "their addition," for since by his evil suggestion he brought into the world the first sin, he never ceases to add to it day by day by prompting to worse things.Or indeed it is in reproach that he is called Leviathan, that is, styled "the addition of men." For he found them immortal in Paradise, but by promising the divine nature to immortal beings, he as it were pledged himself to add somewhat to them beyond what they were. But while with flattering lips he declared that he would give what they did not have, he robbed them cunningly even of what they had. And hence the prophet describes this same Leviathan in these words, "Leviathan, the serpent: even Leviathan that crooked serpent." For this Leviathan crept near to people with tortuous windings through the false promise of what he would give them; for while he falsely promised things impossible, he really stole away even those which were possible.