The marginal reference for subtle is "cunning." "Cunning" is almost always used in a negative sense. Someone who is cunning is skilled in ingenuity or deceit, selfishly clever, crafty. "Cunning" describes those who use their smarts, intelligence, wits to get the best of the other fellow by using whatever deceitful, underhanded means available so they "win."

In the Bible, serpents are depicted as a paradoxical combination of wisdom and evil—beautiful yet repulsive. They have a fluid grace if viewed from a safe distance, but they are to be feared because they strike from hiding places and strike without warning. A serpent symbolizes craftiness that mesmerizes its victims. The contrast with a lamb, a symbol of Jesus Christ, is stunning.