After the rejection of his sacrifice and before the murder of Abel, God offers Cain some advice: If you will improve, you will carry, and if you will not improve, at the doorway sin will crouch. This verse is notoriously unclear. Robert Alter writes that “any construction is no more than an educated guess.” A Koznitzer Rebbe records a teaching of the Baal Shem Tov – if you will improve, you will carry i.e. you will be able to tolerate every type of person. However, if you fail to improve yourself then you will become intolerant and at the doorway sin will crouch – at the door of every other person, all you will be able to see is their faults.