Failure to obey the command specifically requiring a livestock offering rather than produce from already-cursed ground (Genesis 3:17) disqualified Cain's offering. The observation that Cain's countenance fell suggests that he had for some time wallowed in a perpetual state of bitterness, lacking the necessary faith to submit to God's will. Rather than desiring to return to Egypt (symbolic of sin), Cain never left it. Abel, on the other hand, offered his sacrifice in obedience for God's Law. We must worship God on His terms, as He outlines in His Book of Truth (John 4:24). Like Abel, we must believe God and worship Him in a spirit of truth, bringing about a profound change in heart.