In Genesis chapter 4 we read about Cain killing his brother, Abel. As we study this passage we discover that Cain is a picture of man in his natural, unsaved state. Abel is a picture of all true believers. Cain was the firstborn:
"And Adam knew Eve his wife; and she conceived, and bare Cain, and said, I have gotten a man from the LORD." (Genesis 4:1)

But we are going to find out that he was not accepted and, instead, it was the secondborn who received the blessing. This is a principle that can be found throughout the Bible. Abraham’s firstborn, Ishmael, was rejected in favor of his secondborn, Isaac. Isaac’s firstborn, Esau, lost his birthright and blessing and, instead, his secondborn, Jacob, received them. Jacob’s firstborn, Reuben, was rejected in favor of Joseph, his firstborn from his second wife. Joseph’s firstborn, Manasseh, was rejected in favor of his secondborn, Ephraim. We find this principle employed in the kings of Israel as well. The first king, Saul, which was the people’s choice, was rejected in favor of David. In fact, the Bible describes Christ as the last, or second, Adam:
"And so it is written, The first man Adam was made a living soul; the last Adam was made a quickening spirit." (1 Corinthians 15:45)
Therefore, we could say that the first Adam was rejected in favor of the second Adam. We also find this principle in salvation. Our first birth is one that is into a cursed state under the wrath of God. In order to become saved we must be born a second time:
"Jesus answered and said unto him, Verily, verily, I say unto thee, Except a man be born again, he cannot see the kingdom of God." (John 3:3)
This second birth is not a physical birth but a spiritual one:
"Nicodemus saith unto him, How can a man be born when he is old? can he enter the second time into his mother’s womb, and be born? Jesus answered, Verily, verily, I say unto thee, Except a man be born of water and of the Spirit, he cannot enter into the kingdom of God." (John 3:4-5)

Cain was a tiller of the ground while Abel was a shepherd:
"And she again bare his brother Abel. And Abel was a keeper of sheep, but Cain was a tiller of the ground." (Genesis 4:2)
Cain being a tiller of the ground is a reminder of the curse that God placed on mankind for his rebellion:
"And unto Adam he said, Because thou hast hearkened unto the voice of thy wife, and hast eaten of the tree, of which I commanded thee, saying, Thou shalt not eat of it: cursed is the ground for thy sake; in sorrow shalt thou eat of it all the days of thy life; Thorns also and thistles shall it bring forth to thee; and thou shalt eat the herb of the field; In the sweat of thy face shalt thou eat bread, till thou return unto the ground; for out of it wast thou taken: for dust thou art, and unto dust shalt thou return." (Genesis 3:17-19)
Abel being a shepherd is additional evidence that points to the fact that Abel is a representation of all true believers in this historical parable. It’s easy for us to understand how a pastor of a church is a shepherd, but really all believers are shepherds as they bring the Gospel, which God uses to seek and save those who are lost:
"For the Son of man is come to seek and to save that which was lost." (Luke 19:10)

Cain brought some of his crops as an offering to God while Abel brought the best of his flock as an offering. Abel’s offering was accepted by God, but Cain’s was not:
"And in process of time it came to pass, that Cain brought of the fruit of the ground an offering unto the LORD. And Abel, he also brought of the firstlings of his flock and of the fat thereof. And the LORD had respect unto Abel and to his offering: But unto Cain and to his offering he had not respect. And Cain was very wroth, and his countenance fell." (Genesis 4:3-5)
I think to understand why Cain’s offering was not accepted we have to go back to the previous chapter and look at Adam and Eve’s attempt to cover themselves when they realized they were naked. They used fig leaves and we’ve learned that this pointed to mankind’s attempt to cover his spiritual nakedness. However, as we learned, those fig leaves were replaced animal skins that God prepared for them, illustrating that blood must be shed in order to cover our spiritual nakedness:
"And almost all things are by the law purged with blood; and without shedding of blood is no remission." (Hebrews 9:22)
Of course, the blood in view is that of Christ.

Cain brought a bloodless offering while Abel brought an innocent animal to be sacrificed. That animal’s life being given was a picture of Christ, the Lamb of God, being slain as an atonement for the sins of His people. Cain may have brought the finest of his crops, but his offering did not involve the giving of a life. That is a picture of mankind in his natural state who knows there is a God and wants to get right with Him, but he wants to do it without Christ. He doesn’t want to surrender his will to Christ. Instead, he wants to get right with God through his own actions, so he brings the best he can offer. He lives a moral lifestyle while being kind to others. He donates money to the church and reads his Bible every night. These are wonderful things in the world’s eyes, but they do not get us right with God.