“Some would argue that what set Cain's sacrifice apart from Abel’s was not that it was bloodless, but merely that it was insincere- that Cain's heart was not right. If his heart had been in the right place God would have accepted his offering of the fruit of the ground just as He accepted Abel’s lamb. This approach to Cain and Abel's offering begins on rather shaky ground, with the presupposition that what makes worship acceptable or not acceptable to God is the sincerity of the worshipper. Could Cain have offered anything at all to God just as long as he was sincere in his heart? Why then did God reject Nadab and Abihu’s ‘strange fire’? Why did He not accept Uzzah’s sincere desire to steady the Ark?

God is concerned about the sincerity of the worshipper, to be sure. He is not honored by mere forms while the heart of the worshipper is disengaged or sunk in hypocrisy. Nevertheless, sincerity alone is not enough apart from obedience to God's commands…

Cain’s error, then, was far more serious than mere insincerity. His error was that he sought to draw near to God on the basis of the work of his own hands, rather than through substitutionary atonement. The murder of Abel further demonstrates the lengths to which sinful men will go in their rebellion against God. Better to kill the righteous than to submit to God on His terms. No wonder that worship is such a sensitive issue, for it strikes at the very heart of man's sinful nature.” (Comin, 11-12)