Who was Cain in the Bible?

TL;DR:

Cain was the first son of Adam and Eve. He murdered his brother Abel and rejected the opportunity God gave him to turn from his sin. Cain reveals the destruction of sin in our lives and in the lives of others.

what does the bible say?

Cain is the first named son of Adam and Eve, and he is known as the first murderer in the Bible (Genesis 4). He worked the soil while his brother Abel tended sheep. Each of them brought an offering to the Lord, but God rejected Cain’s offering. This angered Cain. God warned Cain about his anger and gave him the opportunity to do what was right, yet Cain instead chose to give in to his anger and murdered his brother. Cain received punishment for his sin including having difficulty working the land and becoming a fugitive. The New Testament speaks of Cain negatively, warning us to not act like Cain. Sin brings destruction to us and to others.

from the old testament

Cain was likely Adam and Eve’s first son (Genesis 4:1).
Cain worked the soil, and his brother Abel was a shepherd (Genesis 4:2).
Cain is the first murderer (Genesis 4:8).
Cain did not offer to the Lord in the way God had instructed him to. Genesis 4:5 tells us "for Cain and his offering [God] had no regard." In contrast, "by faith Abel offered" (Hebrews 11:4) the "firstborn of his flock and of their fat portions" to the Lord, and he had favor with God (Genesis 4:4).
With his offering being rejected, Cain became angry: “So Cain was very angry, and his face fell” (Genesis 4:5).
God exposed Cain’s anger and called him to turn from it: “The LORD said to Cain, 'Why are you angry, and why has your face fallen? If you do well, will you not be accepted? And if you do not do well, sin is crouching at the door. Its desire is contrary to you, but you must rule over it'" (Genesis 4:6–7). Instead of heeding the Lord's warning, Cain acted in anger and murdered Abel (Genesis 4:8).
When God confronted Cain for murdering Abel, Cain responded by asking, “Am I my brother’s keeper?” (Genesis 4:9).
God punished Cain by causing the earth to be even more difficult for him to farm and by banishing him to a life of wandering (Genesis 4:10–12).
Cain was marked by God in some way that ensured he himself would never be murdered (Genesis 4:15).
Cain settled in Nod, a land located east of the garden of Eden, where he built a city and had a family. Cain's descendants are listed in the Bible to the sixth generation. They were smiths, nomadic herdsmen, and musicians (Genesis 4:17–22). Cain's descendants became progressively more wicked and were most likely destroyed in the flood of Noah's day (Genesis 4:23–24).

from the new testament

Hebrews 11:4 seems to indicate that Cain’s offering was not given in faith. Although he offered some of the fruit of the ground to the Lord, it was not an acceptable sacrifice. Without faith we cannot please God (Hebrews 11:6). God calls us to live by faith (Romans 1:17; Galatians 3:11; Hebrews 10:38).
First John 3:12 warns us not to be like Cain, who murdered his brother because his own actions were evil. His actions were a reflection of his heart. Cain welcomed evil into his heart by disobeying God. This led to murder. He had a way out, but he did not choose this way.
Jude 1:10–11 falls within a warning passage about false teachers who "blaspheme all that they do not understand, and they are destroyed by all that they, like unreasoning animals, understand instinctively." These people "walked in the way of Cain." The way of Cain is one of rejecting God and instead choosing what is evil.

implications for today

Cain's life reveals the destruction that sin brings. James tells us that “each person is tempted when he is lured and enticed by his own desire. Then desire when it has conceived gives birth to sin, and sin when it is fully grown brings forth death” (James 1:14–15). When our desires are not rooted in what is good and true, giving in to them, as Cain did, brings destruction to us and to others.

God gives us the opportunity to reject the temptation of our sinful desires. He gave Cain the opportunity to do what is right and to rule over his sinful desires (Genesis 4:7), and He does the same for us. First Corinthians 10:13 tells us, “No temptation has overtaken you that is not common to man. God is faithful, and he will not let you be tempted beyond your ability, but with the temptation he will also provide the way of escape, that you may be able to endure it.” Whenever we are tempted, God gives us the opportunity to turn from it and to do what is right. For believers, the Holy Spirit empowers us to not live according to our sinful nature: “So then, brothers, we are debtors, not to the flesh, to live according to the flesh. For if you live according to the flesh you will die, but if by the Spirit you put to death the deeds of the body, you will live” (Romans 8:12–13). True life is found in living God’s way.

understand

Cain’s anger led to his being the first murderer.
God gave Cain an opportunity to turn from his anger and to control his sin.
Cain rejected God’s offer to resist sin, and his sin impacted him and others.

reflect

Seeing how sin leads to death and destruction, what should be our response to sin?
When tempted, what can we do to see the opportunities God gives us to turn from sin?
Sin begins in the heart and comes out in action. What is the connection between our hearts and our actions?

engage

Some people struggle with God rejecting Cain’s sacrifice and accepting Abel’s because we are not specifically told why God did not accept his sacrifice. Even without details, we know that it had something to do with his faith. God looks at the heart, and our actions reflect the condition of our heart.
God does not tempt anyone (James 1:13), but He allows temptation. Why is this difficult for some to understand or accept?
Living by faith does not mean actions are not important, but it also does not mean that our actions give us faith. Instead, faith produces works in us that reveal our faith. Whether done by faith or in sin, our actions matter and reveal our hearts.