In Genesis 4, Cain and Abel both brought offerings to the LORD. Scripture tells us that Cain brought of the fruit of the ground, which makes sense because he grew crops for a living. Abel, on the other hand, brought one of the firstlings of his flock, which also makes sense as Abel kept sheep for a living.
Now, Genesis 4 tells us that God had respect for Abel’s offering, but not Cain’s. As a result, Cain became very upset. God confronted Cain about his attitude, saying, “If you do well, won’t you be accepted? And if not, sin lies at the door.”
Cain did not do well, however. As he and Abel worked in the field, Cain vented his frustration on Abel, resulting in a fight that led to Cain killing Abel. Cain was subsequently banished, leading him to found a godless society. Adam and Eve had another son, who fathered the Godly lineage in the pre-flood world.
Such set the stage for the downfall of mankind, leading to the flood in Genesis 6-9.

In taking all of this into consideration, the question is often posed, “Why did God respect Abel’s offering, but not Cain’s?”
One answer commonly given is that Abel offered a blood sacrifice, while Cain did not. While this explanation harmonizes with the Gospel pattern of the shedding of blood for the remission of sins, the fact is that there is little Biblical evidence for it, especially when you consider that grain offerings were a completely legitimate part of the temple system of worship in the Old Testament.
The explanation with scriptural evidence is actually much simpler. Abel offered his sacrifice by faith, whereas Cain did not.
Hebrews 11:4 says, “By faith Abel offered unto God a more excellent sacrifice than Cain, by which he obtained witness that he was righteous, God testifying of his gifts: and by it he being dead yet speaketh.”
Abel offered his sacrifice by faith. He knew God was God, trusted in His goodness, and rested in His grace.
On the other hand, Cain lacked this faith. He obviously knew God existed, but did not trust God’s goodness and grace. For Cain, the offering was more of an obligation than an offer of thanksgiving or worship. Thus, God rejected it.
Scripture is very clear, and the point is driven home in Hebrews 11. What pleases God is faith, that simple trust in His existence and grace. While that faith motivates action, the fact is without that faith, the most righteous acts in the world will not please God.
So, with all that in mind, do you have faith. Do you believe that God is? And do you trust in His goodness and grace? (Hebrews 11:6)
And if so, are you living by that faith? May God grant you clarity as you reflect on your faith.