No Natural Difference
Genesis 4: 1-2
Both Cain and Abel were sons of Adam. They were both born with the same corrupt nature which Adam passed on to them in natural generation. Both appear to have been taught by Adam the promise of Christ, which God taught Adam in the garden, because each came to the LORD to make an offering. But Hebrews 11: 4 tells us that there was a difference in these two, "By faith Abel offered unto God a more excellent sacrifice than Cain."