Mormonism is not historic Christianity. When people say, 'Well, Mormons and Christians all believe the same things,' my response is, 'If that's true, why are they always on my front doorstep trying to convert me?'
Abel's sacrifice was accepted by God because it was made according to God's instructions. Cain, however, tried to approach God in his own way and was rejected by God (Genesis 4:3-5). From that point on, Cain became a symbol of those who try to approach God on their own terms.