In the law, which was given long after Cain and Abel, God said there is no remission of sins without the shedding of blood.
However, Paul used the account of Abraham being righteous, before the law was given, because he believed what God said. IMHO, Abel was accepted because his offering was made with blood. In other words Abel believed what God said just as Abraham did in Genius 15. Neither one did any works of the flesh to earn God's favor. That means they had faith in what God said and not in their works of the flesh. ====Faith without works of religion.
Cain refused to do as God said. He, in his heart, insisted that the works of his hands should be accepted by God. His faith was not in God's acceptance but in his works satisfying God. Cain placed his pride in what he did and it was not accepted by God.
If you notice it, in Acts 21:20 the believing Jews were zealous of the law. That means they had pride in their keeping of the Law of Moses. I do not see any words that said they had faith in what Jesus did.
The sin of pride was the sin in Cain's heart and it is the sin that separates men of religion today. Men of religion talk about their keeping of the Law of Moses even though we are not under that law. Wars are fought over religions.
IMHO, this example goes with that of Abraham's example. FAITH IN WHAT GOD SAYS, PLUS NOTHING.
Today we see that Jesus (GOD) gave Paul a hidden gospel of God's grace Man is supposed to believe what Jesus told Paul to preach. A gospel that says Jesus' shed blood has paid for mankind's sins of the flesh. But most religions do not accept Paul's words that his gospel was hidden in God and revealed to him and not the 12. They do not tell others that salvation is a gift from God. They continuously teach the necessity of trying to keep the law. As a person in a Sunday School Class said, " I hope I am good enough to get into heaven. A child of God knows their flesh is not ever good enough but they trust in God who has made away for the ungodly to become the children of God. It is these that give God all the glory.
Acts 22:12-15
12 "Then a certain Ananias, a devout man according to the law, having a good testimony with all the Jews who dwelt there,
13 came to me; and he stood and said to me, 'Brother Saul, receive your sight.' And at that same hour I looked up at him.
14 Then he said,'The God of our fathers has chosen you that you should know His will, and see the Just One, and hear the voice of His mouth.
15 For you will be His witness to all men of what you have seen and heard.
NKJV