"People sometimes say that I am my brother's keeper. What does being our brother's keeper mean, and where does it say that we are supposed to be our brother's keeper?", (Question from James L.).
The phrase brother's keeper comes from Genesis 4.9. After Cain murders Abel, God questions him about his brother, "And the LORD said unto Cain, Where is Abel thy brother? And he said, I know not: Am I my brother's keeper?" (Genesis 4.9 KJV bible). Cain's response here, "Am I my brothers keeper", has become one of the most famous lines in the bible. It is often quoted to imply, that unlike Cain, we are supposed to be our brother's keeper.
Looking closely at Genesis 4.9, we see that Cain's response to God has two parts. First, he claims that he doesn't know where Abel is, which is an obvious lie. Second, Cain answers God with what appears to be a rhetorical question, "am I my brothers keeper". After denying any knowledge of where Abel is, Cain implies that he's not supposed to know where Abel is, because he is not his keeper.
However, there are two questions that still have to be answered with respect to Genesis 4.9. Clearly, Cain is denying responsibility for Abel, but we don't know what level of responsibility he denies by the phrase "brother's keeper". Once we know what is meant by brothers keeper, we need to ask whether Cain's inference that he is not his brother's keeper is incorrect? In other words, should we assume that Genesis 4.9 is teaching us all to be our brother's and sister's keepers?
Given that Cain murders Abel, on account of Abel's righteousness, we know that Cain probably has little concern for anyone besides himself. This leads to the typical interpretation of Genesis 4.9 that Cain is justifying his crime in the belief that he is not his brothers keeper. In asking "am I my brothers keeper", Cain is suggesting that he has no obligation to anyone other than himself. According to this view, God expects that we be our brothers keeper, but Cain assumes the opposite, and so doesn't feel guilty in murdering Abel. Some take this further, and politicize it to say that in God's eyes we are all responsible for the collective well-being of others. If a murderer like Cain suggests he has no responsibility for his fellow man, then concepts such as individualism and personal responsibility must be unbiblical.
However, when we look closely at the phrase "brother's keeper", we find that it means something different from what is often assumed. The Hebrew word that is translated as "keeper" (shaw-mar), means literally a custodian, watcher, or caretaker. So in Cain's response to God, he is literally asking whether he is his brother's guardian or custodian. Perhaps the statement could even be translated "am I my brothers baby-sitter". So the term "keeper" doesn't mean guide, helper, or aid, but implies a much higher level of responsibility, such as a caretaker or parent.
So we find in Cain's question not a defiant belief in individualism or every man for himself. Maybe Cain does believe in these things, but in the context of Genesis 4, Cain's remark is one of sarcasm and absurdity. His younger brother is by this point a grown man, tending flocks of sheep. Of course Cain is not his brother's keeper. No one is their brother's or sister's keeper (shawmar), unless that person is incapable of taking care of him or herself.
By knowing the meaning of the term brother's keeper, we can better understand what takes place in Genesis 4.9. In questioning Cain about Abel, God alludes that he might know something about what has happened to him. Cain first denies any knowledge of his brothers disappearance. Then he adds in a sarcastic rhetorical question meant to deflect away any further inquiry (leave it to Cain to be sarcastic with God). Clearly the truth makes Cain uncomfortable, and he doesn't want to face up to what he has done. Therefore, Cain is not trying to justify his evil deed by saying he is not his brother's keeper, but to deflect and avoid the consequences of it.
Under normal circumstances we are not the literal or figurative keepers of our brothers and sisters. However, as Christians there are responsibilities that we have to our fellow man, "And the second is like unto it, Thou shalt love thy neighbour as thyself" (Matthew 22.39 KJV bible). Loving thy neighbor as thyself doesn't mean being your neighbor's keeper or overseer. Instead it means taking his or her best interests to heart. Sometimes this means giving to others, or inconveniencing ourselves, and other times it doesn't. Thus, as Christians our responsibility is to empathize with others, and consider their well-being as our own.
More important than the secular welfare of others, is their spiritual welfare. As Christians our most important responsibility is to expose others to the truth, "When I say unto the wicked, O wicked man, thou shalt surely die; if thou dost not speak to warn the wicked from his way, that wicked man shall die in his iniquity; but his blood will I require at thine hand. Nevertheless, if thou warn the wicked of his way to turn from it; if he do not turn from his way, he shall die in his iniquity; but thou hast delivered thy soul" (Ezekiel 33.8-9 KJV bible).
As Christians we must act as spiritual watchmen. We need to declare the reality of salvation and God's future wrath upon the world. Like a watchman, we are not accountable for how others react to our declaration. Neither is it our place to be overseers and force them to believe in a certain ways. However, we are responsible for sounding the trumpet and ensuring that as many people as possible are exposed to the Word. Thus, we are not responsible for the choices of our fellow men, but we have responsiblities to them.