Cain's attempt at salvation was by the works of his own hands from tilling the Land
And in process of time it came to pass, that Cain brought of the fruit of the ground an offering unto the LORD. Genesis 4:3
Abel's attempt at salvation was obedience to God and the sacrifice of the Lamb
Abel, he also brought of the firstlings of his flock and of the fat thereof. And the LORD had respect unto Abel and to his offering: Genesis 4:4