The very first conflict following the Fall was a conflict over worship.
Genesis 4:3–8 relates how Abel’s offering to the Lord’s was accepted, while Cain’s was not. These offerings were important because they were God’s means for at least temporarily and partially restoring communion with his people. Yet for some reason that is not explicit in the text, God rejected Cain’s offering. At this point in Genesis we do not have any clear revelation as to the content of acceptable worship. It is at least possible that God had demanded an animal sacrifice, and since Cain brought a food offering instead, that is why God rejected him. There is no way to determine this with absolute certainty, however. On the other hand, some assume that the offering wasn’t the problem; Cain’s heart attitude or motivation were deficient, and that is why God rejected him. This conclusions, too, is based on speculation since the text does not fully explain why God rejected Cain.
Revelation in the New Testament, however, does give a clearer picture. The author of Hebrews explains that Abel proved that he was “righteous” through his sacrifice, while Jude 11 describes Cain as one who “abandoned himself for the sake of gain,” and 1 John 3:12 says that Cain’s deeds were “evil.” These descriptions relate most specifically to the inward spiritual condition of the men, yet Hebrews 11:4 also notes that Abel’s sacrifice itself was “more excellent” than Cain’s and that his righteousness was determined by “his gifts.”
Taken together, this information reveals a two-fold emphasis in evaluating worship, an emphasis that appears often in biblical discussions of worship.
First, the inward heart motivation is of utmost importance in true worship. At least part of what led to Cain’s rejection was an evil heart.
But a heart of worship will manifest itself in excellent, obedient offerings to God. Good heart motivation alone does not justify disobedience of God’s clear instructions or worshiping in flippant, casual ways. On the other hand, doing exactly what God has commanded without a heart that desires to please and glorify him is equally deficient.
Both are important.