"If you do well, will you not be accepted? And if you do not do well, sin is crouching at the door. Its desire is for you, but you must rule over it."
I have recently been devoting much time and energy to deeply considering what this means. This was God's most intimate advice to Cain upon the most disappointing failure of his life. Cain was presented with a clear choice. Either he could allow bitterness and resentment to become a full blown, murderous impulse, or, he could choose to "rule over" the sin that was threatening to overtake him.
I think that sin is most easy to rule over when the inclination has just begun, before it begins to grow. If we can manage to simply control our thoughts and impulses at that initial stage, sin can be vanquished. It is when we entertain those thoughts that we begin to loose control of ourselves and give into sin. And small sins, when surrendered to, almost always blossom into very large sins.
What began in Cain as profound disappointment and jealousy blossomed into murder. And yet, God said that it was possible for him to rule over it.
Perhaps that is one of the reasons that God allows us to experience failure in the midst of life. He already knows whether or not we will surrender to sin. We need the opportunity to be tested and to come to know this about ourselves. It is of critical importance that we learn to rule over our own sin. If not, it will rule over us.