"And in process of time it came to pass, that Cain brought of the fruit of the ground an offering unto the LORD. And 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: but unto Cain and to his offering he had not respect. And Cain was very wroth, and his countenance fell" (Gen. 4:3-5). What was it regarding Cain's offering that made it unacceptable before God? The preference for Abel's offering and the rejection of Cain's was not arbitrary, but based upon past revelation given to Adam and his family. Evidently, God revealed this information to Adam when He killed animals to make coverings for Adam and his wife (Gen. 3:21). Generations later, Noah knew that God would only accept clean animals and birds as burnt offerings to the Lord (Gen. 8:20). Cain, unlike his brother Abel, decided, apart from God's Word, that an offering of the fruit of the ground would be acceptable before the Lord. But God rejected Cain's offering, because it was a creation of his mind. God did not command it; therefore, even if Cain was sincere in his desire to please God, God still would have rejected his offering. God expects faith and obedience to His Word. If God's people can worship the Lord according to their own will, as long as the man-made ordinances are not expressly forbidden, then could not Cain, Noah or the Levites offer God a fruit salad or a bucket of turnips, for it is nowhere forbidden? And if God wanted a strict regulation of His worship apart from the regulative principle, would it not require hundreds (or perhaps thousands) of volumes telling us what is forbidden? But God, in His infinite wisdom, says, "What thing soever I command you, observe to do it: thou shalt not add thereto, nor diminish from it" (Deut. 12:32).