Sin

When Bilbo gives up the ring, Gandalf hesitates to pick it up himself, and leaves that to Frodo. He senses it might be the one ring, and is wise enough to suspect that if that is the case, he might not be able to withstand the temptation to keep it. Gandalf is disciplined and experienced enough, and does not take the bait.

All the way in the beginning of the Bible, in Genesis 4:7, God warns Cain about sin crouching at the door: it makes itself look smaller than it is. Every important decision you makes involves underlying principles.

The question is: will you rule over it, or will you let it rule you?