Adam toiled in sweat, as we read in Genesis 3, but he still ate of the field. However, God said Cain wouldn’t receive food grown to the extent Adam did. This is important for two reasons.
First, God punished Cain by removing his livelihood. It’s a logical consequence that often comes today, too, though not everyone receives the same, because God’s mercy reaches everyone. Justice sometimes isn’t served till eternity, either, because this world isn’t perfect. God gives people freedom so they won’t be mindless robots.
More importantly, people need food to survive. Cain would have to rely on someone else. Cain’s problem was that he relied on his own means to reach God. Now, Cain would need to learn to rely on others just to survive. The end result was that he would hopefully learn to rely on God. The Lord hoped that this would teach him a need to rely on the Lord for everything.
God also shows that He knows what this will mean. First, Cain would be a fugitive. This is further evidence that there were others on the Earth at this time, as it is doubtful He only refers to his parents; though they clearly suffered greatly, too. One son was a murderer, and another son had been killed! Everyone would learn what had happened to their brother Abel. And, they would hate Cain for it. He might find someone willing to care, but in general, he would fear reprisal. Therefore, he would be a vagabond, wandering from place to place.
These, too, were supposed to teach Cain to rely on the Lord. Adam and Eve had the entire Earth all to themselves at first, so they saw how insignificant they were, and how much they needed to rely on God. Now, Cain found himself in a similar situation. He would have to fend for himself, and he would hopefully turn to God for help. Sadly, there is no evidence Cain ever turned to the Lord.
