The peace offering teaches many things, but one of its main symbols is fellowship. Our communion with the Father and the Son obligates us to pursue peace.
Cain represents religion and worship on a person's own terms, according to his own priorities, rather than according to God's instruction.
Why do so many nominal Christians reject works and obedience to God's law? Largely because they fail to gather God's whole counsel on this subject.
The meal offering represents the second Great Commandment, love toward fellow man. Our service to others requires much grinding self-sacrifice and surrender.
Ancient Israel had at the core of its religion an obsession to please the self at the expense of justice and the best interests of the disadvantaged.
Israel had every opportunity that the Gentiles did not have. God gave the Israelites gifts to live a better way, but they completely failed to reflect Him.