God's having made for man coats of skin presupposes the slaying of animals; and doubtless implies that Abel's sacrifice of an animal life was an act of faith which rested on God's command (though not expressly recorded) that such were the sacrifices He required.
If it had not been God's command, it would have been presumptuous will worship (Colossians 2:23), and taking of a life which man had no right over before the flood (Genesis 9:2-4). Cain in self-righteous unbelief, refusing to confess his guilt and need of atonement (typified by sacrifice), presented a mere thank offering of the first fruits; not, like Abel, feeling his need of the propitiatory offering for sin.
- M.O. Evans