The text then records God's warning to Cain and his disregard for that warning. Cain was angry with God and he gave in to the temptation to take it out on his brother. "Cain rose up against Abel his brother and killed him" (Gen. 4:8). When God asked Cain about Abel, he lied in affirming that he didn't know where Abel was. He went on to ask - "Am I my brother's keeper?" (Gen. 4:9).
This is a question we ought to ask ourselves. Am I supposed to be my "brother's keeper"? The biblical answer is "yes." I should be concerned about others and not just myself. I should be concerned about my brethren in the church, for we are members of the same body. We should have the same goal of serving the Lord faithfully so we may go home to be with Him one day. I should have other Christians' best interests at heart always (cf. I Cor. 10:24). Envy should not be allowed a place in my heart. Likewise, my brothers in Christ should also be looking out for me and my soul.
Let me hasten to point out that being my "brother's keeper" is not limited to just my fellow Christians. In a physical sense, all human beings are related. We all have the same heavenly Father. Thus, we are all family or "brothers." Although we should give extra care and attention to fellow Christians, we ought to be looking out for the best interests of those in the world also. As Galatians 6:10 says - "Therefore, as we have opportunity, let us do good to all, especially to those who are of the household of faith." And what is in the best interest of those in the world? Is it not that they hear the gospel and see Jesus living in me through my speech and actions? Indeed. When the Lord commanded His disciples to go, teach, and make disciples (Matt. 28:19,20), He was instructing His followers to be their "brother's keeper."
Ultimately, the best answer to Cain's question is found in Matthew 7:12, which we commonly refer to as "the golden rule": do unto others as you would have them do unto you. Do I desire that others watch out for my best interests and help me on my journey to heaven? Absolutely. Then, I should do likewise for them. Faithfully applying the golden rule demands that I be my "brother's keeper."
Dear listeners, as Cain well illustrates, no one can be their "brother's keeper" with envy or hatred in their heart. If you've allowed such emotions to reside within you, remove them, and endeavor to actively seek the best for others, watching and caring for their souls as you do your own.